Margulis Lemma
Margulis Lemma
Margulis Lemma
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STRUCTURE OF FUNDAMENTAL GROUPS OF MANIFOLDS<br />
WITH RICCI CURVATURE BOUNDED BELOW<br />
VITALI KAPOVITCH AND BURKHARD WILKING<br />
The main result of this paper is the following theorem which settles a conjecture<br />
of Gromov.<br />
Theorem 1 (Generalized <strong>Margulis</strong> <strong>Lemma</strong>). In each dimension n there are positive<br />
constants C(n), ε(n) such that the following holds for any complete n-dimensional<br />
Riemannian manifold (M, g) with Ric > −(n − 1) on a metric ball B 1 (p) ⊂ M.<br />
The image of the natural homomorphism<br />
π 1<br />
(<br />
Bε (p), p ) → π 1<br />
(<br />
B1 (p), p )<br />
contains a nilpotent subgroup N of index ≤ C(n). Moreover, N has a nilpotent basis<br />
of length at most n.<br />
We call a generator system b 1 , . . . , b n of a group N a nilpotent basis if the commutator<br />
[b i , b j ] is contained in the subgroup 〈b 1 , . . . , b i−1 〉 for 1 ≤ i < j ≤ n.<br />
Having a nilpotent basis of length n implies in particular rank(N) ≤ n. We will<br />
also show that equality in this inequality can only occur if M is homeomorphic to<br />
an infranilmanifold, see Corollary 7.1.<br />
In the case of a sectional curvature bound the theorem is due to Kapovitch,<br />
Petrunin and Tuschmann [KPT10], based on an earlier version which was proved<br />
by Fukaya and Yamaguchi [FY92].<br />
In the case of Ricci curvature a weaker form of the theorem was stated by<br />
Cheeger and Colding [CC96]. The main difference is that – similar to Fukaya and<br />
Yamaguchi’s theorem – no uniform bound on the index of the subgroup is provided.<br />
Cheeger and Colding never wrote up the details of the proof, and relied on some<br />
claims in [FY92] stating that their results would carry over from lower sectional<br />
curvature bounds to lower Ricci curvature bounds if certain structure results would<br />
be obtained. But compare also Remark 6.2 below.<br />
Corollary 2. Let (M, g) be a compact manifold with Ric > −(n−1) and diam(M) ≤<br />
ε(n) then π 1 (M) contains a nilpotent subgroup N of index ≤ C(n). Moreover, N<br />
has a nilpotent basis of length ≤ n.<br />
One of the tools used to prove these results is<br />
Theorem 3. Given n and D there exists C such that for any n-manifold with<br />
Ric ≥ −(n−1) and diam(M, g) ≤ D, the fundamental group π 1 (M) can be generated<br />
by at most C elements.<br />
2000 Mathematics Subject classification. Primary 53C20. Keywords: almost nonnegative Ricci<br />
curvature, <strong>Margulis</strong> <strong>Lemma</strong>.<br />
The first author was supported in part by a Discovery grant from NSERC.<br />
1
2 VITALI KAPOVITCH AND BURKHARD WILKING<br />
This estimate was previously proven by Gromov [Gro78] under the stronger<br />
assumption of a lower sectional curvature bound K ≥ −1. Recall that a conjecture<br />
of Milnor states that the fundamental group of an open manifold with nonnegative<br />
Ricci curvature is finitely generated. Although Theorem 3 is far from a solution to<br />
that problem, the <strong>Margulis</strong> <strong>Lemma</strong> immediately implies the following.<br />
Corollary 4. Let (M, g) be an open n-manifold with nonnegative Ricci curvature.<br />
Then π 1 (M) contains a nilpotent subgroup N of index ≤ C(n) such that any finitely<br />
generated subgroup of N has a nilpotent basis of length ≤ n.<br />
Of course, by work of Milnor [Mil68] and Gromov [Gro81], the corollary is well<br />
known (without uniform bound on the index) in the case of finitely generated<br />
fundamental groups. We should mention that by work of Wei [Wei88] (and an<br />
extension in [Wil00]) every finitely generated virtually nilpotent group appears<br />
as fundamental group of an open manifold with positive Ricci curvature in some<br />
dimension.<br />
The proofs of our results are based on the structure results of Cheeger and<br />
Colding for limit spaces of manifolds with lower Ricci curvature bounds [Col96,<br />
Col97, CC96, CC97, CC00a, CC00b]. This is also true for the proof of following<br />
new tool which can be considered as a Ricci curvature replacement of Yamaguchi’s<br />
fibration theorem as well as a replacement of the gradient flow of semi-concave<br />
functions used by Kapovitch, Petrunin and Tuschmann.<br />
Theorem 5 (Rescaling theorem). Suppose a sequence of Riemannian n-manifolds<br />
(M i , p i ) with Ric ≥ − 1 i<br />
converges in the pointed Gromov–Hausdorff topology to the<br />
Euclidean space (R k , 0) with k < n. Then after passing to a subsequence there is a<br />
subset G 1 (p i ) ⊂ B 1 (p i ), a rescaling sequence λ i → ∞ and a compact metric space<br />
K ≠ {pt} such that<br />
• vol(G 1 (p i )) ≥ (1 − 1 i ) vol(B 1(p i ))<br />
• For all x i ∈ G 1 (p i ) the isometry type of any limit (λ i M i , x i ) is given by<br />
K × R k .<br />
• For all x i , y i ∈ G 1 (p i ) we can find a diffeomorphism f i : M i → M i such<br />
that f i subconverges in the weakly measured sense to an isometry<br />
f ∞ :<br />
lim (λ iM i , x i ) → lim (λ iM i , y i ).<br />
G-H,i→∞ G-H,i→∞<br />
We will prove a technical generalization of this theorem in section 5. It will be<br />
important for the proof of the <strong>Margulis</strong> <strong>Lemma</strong> that the diffeomorphisms f i are<br />
in a suitable sense close to isometries on all scales. The precise concept is called<br />
zooming in property and is defined in section 3.<br />
The diffeomorphisms f i will be composed out of gradient flows of harmonic<br />
functions arising in the analysis of Cheeger and Colding. Their L 2 -estimates on the<br />
Hessian’s of these functions play a crucial role.<br />
Like in Fukaya and Yamaguchi’s paper the idea of the proof of the <strong>Margulis</strong><br />
<strong>Lemma</strong> is to consider a contradicting sequence (M i , g i ). By a fundamental observation<br />
of Gromov, the set of complete n-manifolds with Ric ≥ −(n − 1) is<br />
precompact in Gromov–Hausdorff topology. Therefore one can assume that the<br />
contradicting sequence converges to a (possibly very singular) space X. One then<br />
uses various rescalings and normal coverings of (M i , g i ) in order to find contradicting<br />
sequences converging to higher and higher dimensional spaces. One of the<br />
differences to Fukaya and Yamaguchi’s approach is that if we pass to a normal
STRUCTURE OF FUNDAMENTAL GROUPS 3<br />
covering we endow the cover with generators of the deck transformation group in<br />
order not to loose information. Since after rescaling the displacements of these<br />
deck transformations converge to infinity, this approach seems bound to failure.<br />
However, using the rescaling theorem we are able to alter the deck transformations<br />
by composing them with a sequence of diffeomorphisms which are isotopic to the<br />
identity and which have the zooming in property. With these alterations we are<br />
able to keep much more information on the action by conjugation of long homotopy<br />
classes on very short ones. Since the altered deck transformations still converge in<br />
a weakly measured sense to isometries, this allows one to eventually rule out the<br />
existence of a contradicting sequence.<br />
For the proof of the <strong>Margulis</strong> <strong>Lemma</strong> we do not need any sophisticated structure<br />
results on the isometry group of the limit space except for the relatively elementary<br />
Gap <strong>Lemma</strong> 2.4 guaranteeing that generic orbits of the limit group are locally path<br />
connected. However, for the following application of the <strong>Margulis</strong> <strong>Lemma</strong> a recent<br />
structure result of Colding and Naber [CN10] is key. They showed that the isometry<br />
group of a limit space with lower Ricci curvature bound has no small subgroups and<br />
thus, by the small subgroup theorem of Gleason, Montgomery and Zippin [MZ55],<br />
is a Lie group.<br />
Theorem 6 (Compact Version of the <strong>Margulis</strong> <strong>Lemma</strong>). Given n and D there are<br />
positive constants ε 0 and C such that the following holds: If (M, g) is a compact<br />
n-manifold M with Ric > −(n − 1) and diam(M) ≤ D, then there is ε ≥ ε 0 and a<br />
normal subgroup N ⊳ π 1 (M) such that for all p ∈ M:<br />
• the image of π 1 (B ε/1000 (p), p) → π 1 (M, p) contains N,<br />
• the index of N in the image of π 1 (B ε (p), p) → π 1 (M, p) is ≤ C and<br />
• N is a nilpotent group which has a nilpotent basis of length ≤ n.<br />
If D, n, ε 0 and C are given, there is an effective way to limit the number of<br />
possibilities for the quotient group π 1 (M, p)/N, see <strong>Lemma</strong> 9.2. In fact we have the<br />
following finiteness result. We recall that the torsion elements Tor(N) of a nilpotent<br />
group N form a subgroup.<br />
Theorem 7. a) For each D > 0 and each dimension n there are finitely<br />
many groups F 1 , . . . , F k such that the following holds: If M is a compact n-<br />
manifold with Ric > −(n − 1) and diam(M) ≤ D, then there is a nilpotent<br />
normal subgroup N ⊳ π 1 (M) with a nilpotent basis of length ≤ n − 1 and<br />
rank(N) ≤ n − 2 such that π 1 (M)/N ∼ = F i for suitable i.<br />
b) In addition to a) one can choose a finite collection of irreducible rational<br />
representations ρ j i : F i → GL(n j i , Q) (j = 1, . . . , µ i, i = 1, . . . , k) such that<br />
for a suitable choice of the isomorphism π 1 (M)/N ∼ = F i the following holds:<br />
There is a chain of subgroups Tor(N) = N 0 ⊳ · · · ⊳ N h0 = N which are all<br />
normal in π 1 (M) such that [N, N h ] ⊂ N h−1 and N h /N h−1 is free abelian.<br />
Moroever, the action of π 1 (M) on N by conjugation induces an action of F i<br />
on N h /N h−1 and the induced representation ρ: F i → GL ( (N h /N h−1 ) ⊗ Z Q )<br />
is isomorphic to ρ j i for a suitable j = j(h), h = 1, . . . , h 0.<br />
Part a) of Theorem 7 generalizes a result of Anderson [And90] who proved finiteness<br />
of fundamental groups under the additional assumption of a uniform lower<br />
bound on volume.<br />
In the preprint [Wil11] a partial converse of Theorem 7 is proved. In particular<br />
it is shown there that for a finite collection of finitely presented groups and any
4 VITALI KAPOVITCH AND BURKHARD WILKING<br />
finite collection of rational representations one can find D such that each group Γ<br />
satisfying the algebraic restrictions described in a) and b) of the above theorem<br />
with respect to this data contains a finite nilpotent normal subgroup H such that<br />
Γ/H can be realized as a fundamental group of a n + 2-dimensional manifold with<br />
diam(M) ≤ D and sectional curvature |K| ≤ 1.<br />
We can also extend the diameter ratio theorem of Fukaya and Yamaguchi [FY92].<br />
Theorem 8 (Diameter Ratio Theorem). For n and D there is a ˜D such that any<br />
compact manifold M with Ric ≥ −(n − 1) and diam(M) = D satisfies: If π 1 (M) is<br />
finite, then the diameter of the universal cover ˜M of M is bounded above by ˜D.<br />
In the case of nonnegative Ricci curvature the theorem says that the ratio<br />
diam( ˜M)/ diam(M) is bounded above. Fukaya and Yamaguchi’s theorem covers<br />
the case that M has almost nonnegative sectional curvature. The proof of Theorem<br />
8 has some similarities to parts of the proof of Gromov’s polynomial growth<br />
theorem [Gro81].<br />
Part of the paper was written up while the second named author was a Visiting<br />
Miller Professor at the University of California at Berkeley. He would like to thank<br />
the Miller institute for support and hospitality.<br />
Organization of the Paper<br />
1. Prerequisites 5<br />
2. Finite generation of fundamental groups 9<br />
3. Maps which are on all scales close to isometries 13<br />
4. A rough idea of the proof of the <strong>Margulis</strong> <strong>Lemma</strong> 20<br />
5. The Rescaling Theorem 21<br />
6. The Induction Theorem for C-Nilpotency 26<br />
7. <strong>Margulis</strong> <strong>Lemma</strong> 31<br />
8. Almost nonnegatively curved manifolds with b 1 (M, Z p ) = n 35<br />
9. Finiteness Results 37<br />
10. The Diameter Ratio Theorem 44<br />
We start in section 1 with prerequisites. We have included a subsection on<br />
notational conventions. Next, in section 2 we will prove Theorem 3.<br />
Section 3 is somewhat technical. We define the zooming in property, prove the<br />
needed properties, and provide two somewhat similar construction methods. This<br />
section serves mainly as a preparation for the proof of the rescaling theorem.<br />
We have added a short section 4 in which we sketch a rough idea of the proof of<br />
the <strong>Margulis</strong> <strong>Lemma</strong>.<br />
In section 5 the refined rescaling theorem (Theorem 5.1) is stated and proven.<br />
In Section 6 we put things together and provide a proof of the Induction Theorem,<br />
which has Corollary 2 as its immediate consequence.<br />
The <strong>Margulis</strong> <strong>Lemma</strong> follows from the Induction Theorem in two steps which are<br />
somewhat similar to Fukaya and Yamaguchi’s approach. Nevertheless, we included<br />
all details in section 7. We also show that the nilpotent group N in the <strong>Margulis</strong><br />
<strong>Lemma</strong> can only have rank n if the underlying manifold is homeomorphic to an<br />
infranilmanifold.<br />
Section 8 uses lower sectional curvature bounds. We give counterexamples to a<br />
theorem of Fukaya and Yamaguchi [FY92] stating that almost nonnegatively curved<br />
n-manifolds with first Z p -Betti number equal to n have to be tori, provided p is
STRUCTURE OF FUNDAMENTAL GROUPS 5<br />
sufficiently big. We show that instead these manifolds are nilmanifolds and that<br />
every nilmanifold covers another nilmanifold with maximal first Z p Betti number.<br />
Section 9 contains the proof of Theorem 6 and Theorem 7. Finally, Theorem 8<br />
is proved in section 10.<br />
1. Prerequisites<br />
1.1. Short basis. We will use the following construction due to Gromov.<br />
Given a manifold (M, p) and a group Γ acting properly discontinuously and<br />
isometrically on M one can define a short basis of the action of Γ at p as follows:<br />
For γ ∈ Γ we will refer to |γ| = d(p, γ(p)) as the norm or the length of γ. Choose<br />
γ 1 ∈ Γ with the minimal norm in Γ. Next choose γ 2 to have minimal norm in<br />
Γ\〈γ 1 〉. On the n-th step choose γ n to have minimal norm in Γ\〈γ 1 , γ 2 , ..., γ n−1 〉.<br />
The sequence {γ 1 , γ 2 , ...} is called a short basis of Γ at p. In general, the number of<br />
elements of a short basis can be finite or infinite. In the special case of the action<br />
of the fundamental group π 1 (M, p) on the universal cover ˜M of M one speaks of<br />
the short basis of π 1 (M, p). For any i > j we have |γ i | ≤ |γ −1<br />
j γ i |.<br />
While a short basis need not be unique the non-uniqueness will not matter in<br />
any of the proofs in this article and will largely be suppressed.<br />
If M/Γ is a closed manifold, then the short basis is finite and |γ i | ≤ 2 diam(M/Γ).<br />
1.2. Ricci curvature. Throughout this paper we will use the notation − ∫ to denote<br />
the average integral. We will use the following results of Cheeger and Colding.<br />
Theorem 1.1. [CC96] Let (M n i , p i) G−H<br />
−→<br />
i→∞ (X, p) with Ric(M i) ≥ − 1 i . Suppose X<br />
has a line. Then X splits isometrically as X ∼ = Y × R.<br />
We will also need the following corollary of the stability theorem.<br />
Theorem 1.2. [CC97] Let (Mi n, p i) → (R n , 0) with Ric Mi > −1. Then B R (p i ) is<br />
contractible in B R+ε (p i ) for all i ≥ i 0 (R, ε).<br />
Even more important for us is the following theorem which is closely linked with<br />
the proof of the splitting theorem for limit spaces.<br />
Theorem 1.3. [CC00a] Suppose (Mi n, p i) → (R k , 0) with Ric Mi ≥ −1/i. Then<br />
there exist harmonic functions b i 1, . . . , b i k : B 2(p i ) → R such that<br />
(1) |∇b i j| ≤ C(n) for all i and j and<br />
∫<br />
(2) −<br />
B 1(p i)<br />
∑<br />
∣ ∑<br />
∣ < ∇b<br />
i<br />
j , ∇b i l > −δ j,l + ‖Hess b i<br />
j<br />
‖ 2 dµ → 0 as i → ∞.<br />
j,l<br />
Moreover, the maps Φ i = (b i 1, . . . , b i k ): M i → R k provide ε i -Gromov–Hausdorff<br />
approximations between B 1 (p i ) and B 1 (0) with ε i → 0.<br />
The functions b i j in the above theorem are constructed as follows. Approximate<br />
Busemann functions f j in R k given by f j = d(·, N i e j )) − N i are lifted to M i using<br />
Hausdorff approximations to corresponding functions fj i. Here e j is the j-th coordinate<br />
vector in the standard basis of R k and N i → ∞ sufficiently slowly so that<br />
j
6 VITALI KAPOVITCH AND BURKHARD WILKING<br />
d G−H (B Ni (p i ), B Ni (0)) → 0 as i → ∞. The functions b i j are obtained by solving<br />
the Dirichlet problem on B 2 (p i ) with b i i | ∂B 2(p i ) = fi i| ∂B 2(p i )<br />
We will need a weak type 1-1 estimate for manifolds with lower Ricci curvature<br />
bounds which is a well-known consequence of the doubling inequality [Ste93, p. 12].<br />
<strong>Lemma</strong> 1.4 (Weak type 1-1 inequality). Suppose (M n , g) has Ric ≥ −1 and let<br />
f : M → R be a nonnegative function. Define Mx f(p) := sup r≤1 − ∫ B r(p)<br />
f. Then<br />
the following holds<br />
(a) If f ∈ L α (M) with α ≥ 1 then Mx f is finite almost everywhere.<br />
(b) If f ∈ L 1 (M) then vol { x | Mx f(x) > c } ≤ C(n) ∫<br />
c<br />
f for any c > 0.<br />
M<br />
(c) If f ∈ L α (M) with α > 1 then Mx f ∈ L α (M) and || Mx f|| α ≤ C(n, α)||f|| α .<br />
Note that as an immediate corollary we see that if f ∈ L α (M) with α > 1 then<br />
for any x ∈ M we have the following pointwise estimate<br />
(3) Mx((Mx f) α )(x) ≤ C(n, α) Mx(f α )(x).<br />
We will also need the following inequality (for any α > 1) which is an immediate<br />
consequence of the definition of Mx and Hölder inequality.<br />
(4) [Mx f(x)] α ≤ Mx(f α )(x).<br />
Indeed, for any r ≤ 1 we have that Mx(f α )(x) ≥ − ∫ B r(x) f α ≥ ( − ∫ B r(x) f) α<br />
and<br />
the inequality follows by taking the supremum over all r ≤ 1.<br />
Combining the last two inequalities we deduce<br />
(5) Mx[Mx(f)](x) ≤ Mx[(Mx(f)) α ] 1/α (x) ≤ C(n, α) 1/α Mx[f α ](x)<br />
for α > 1. Finally we note that for α > 1, β > 1 we have<br />
(6) Mx[Mx(f β/α )] α (x) ≤ C(n, α) Mx(f β )(x).<br />
This inequality immediately follows by applying (3) to g = f β α .<br />
We will also make use of the so-called segment inequality of Cheeger and Colding<br />
which says that in average the integral of a nonnegative function along all geodesics<br />
in ball can be estimated by the L 1 norm of the function.<br />
Theorem 1.5 (Segment inequality). [CC96] Given n and r 0 there exists τ =<br />
τ(n, r 0 ) such that the following holds. Let Ric(M n ) ≥ −(n−1) and let g : M → R +<br />
be a nonnegative function. Then for r ≤ r 0<br />
∫<br />
−<br />
B r(p)×B r(p)<br />
∫ d(z1,z 2)<br />
0<br />
∫<br />
g(γ z1,z 2<br />
(t))dµ(z 1 )dµ(z 2 ) ≤ τ · r · − g(q) dµ(q),<br />
B 2r(p)<br />
where γ z1,z 2<br />
denotes a minimal geodesic from z 1 to z 2 .<br />
Finally we will need the following observation<br />
<strong>Lemma</strong> 1.6 (Covering <strong>Lemma</strong>). There exists a constant C(n) such that the following<br />
holds. Suppose (M n , g) has Ric g ≥ −(n − 1). Let f : M → R be a nonnegative<br />
function, p ∈ M, π : ˜M → M be the universal cover of M, ˜p ∈ ˜M a lift of p, and<br />
let ˜f = f ◦ π. Then ∫<br />
∫<br />
− ˜f ≤ C(n)− f.<br />
B 1(˜p)<br />
B 1(p)
STRUCTURE OF FUNDAMENTAL GROUPS 7<br />
Proof. We choose a measurable section j : B 1 (p) → B 1 (˜p), i.e. π(j(x)) = x for any<br />
x ∈ B 1 (p). Let T = j(B 1 (p)). Then we obviously have that diam(T ) ≤ 2 and<br />
∫ ∫<br />
− f = − ˜f.<br />
B 1(p)<br />
Let S be the union of g(T ) over all g ∈ π 1 (M) such that g(T ) ∩ B 1 (˜p) ≠ ∅. It’s<br />
obvious from the triangle inequality that S ⊂ B 3 (˜p). It is also clear that<br />
∫ ∫<br />
− f = − ˜f<br />
B 1(p)<br />
and B 1 (˜p) ⊂ S. Lastly, notice that by Bishop–Gromov relative volume comparison<br />
vol B 3 (˜p) ≤ C(n) vol B 1 (˜p) for some universal constant C(n). Since B 1 (˜p) ⊂ S ⊂<br />
B 3 (˜p), we also have vol B 3 (˜p) ≤ C(n) vol S and hence<br />
∫<br />
∫ ∫<br />
− ˜f ≤ C(n)− ˜f = C(n)− f.<br />
B 1(˜p)<br />
S<br />
T<br />
S<br />
B 1(p)<br />
It is easy to see that the above proof generalizes to an arbitrary cover of M.<br />
1.3. Equivariant Gromov–Hausdorff convergence. Let Γ i be a sequence of<br />
closed subgroups of the isometry groups of metric spaces X i and ˜p i ∈ X i . If<br />
(X i , ˜p i ) converges in the pointed Gromov–Hausdorff topology to (Y, ˜p ∞ ), then after<br />
passing to a subsequence one can find a closed subgroup G ⊂ Iso(Y ) such that<br />
(X i , Γ i , ˜p i ) → (Y, G, ˜p ∞ ) in the equivariant Gromov–Hausdorff topology. For details<br />
we refer the reader to [FY92].<br />
Definition 1.7. A sequence of subgroups Υ i ⊂ Γ i is called uniformly open, if there<br />
is some ε > 0 such that Υ i contains the set<br />
{<br />
g ∈ Γi | d(gq, q) < ε for all q ∈ B 1/ε (˜p i ) } for all i.<br />
Γ i is called uniformly discrete if {e} ⊂ Γ i is uniformly open. We say the sequence<br />
Υ i is boundedly generated if there is some R such that Υ i is generated by<br />
{g ∈ Υ i | d(g˜p i , ˜p i ) < R}.<br />
<strong>Lemma</strong> 1.8. Let Υ j i ⊂ Γ i be uniformly open with Υ j i → Υj ∞ ⊂ G, j = 1, 2.<br />
a) Υ 1 i ∩ Υ2 i is uniformly open and converges to Υ1 ∞ ∩ Υ 2 ∞.<br />
b) If g i ∈ Γ i converges to g ∞ ∈ G then g i Υ 1 i g−1 i is uniformly open and converges<br />
to g ∞ Υ 1 ∞g∞ −1 .<br />
c) Suppose in addition that Υ j i is boundedly generated, j = 1, 2. If Υ1 ∞ ∩ Υ 2 ∞<br />
has finite index H in Υ 1 ∞, then Υ 1 i ∩ Υ2 i has index H in Υ1 i for all large i.<br />
The proof is an easy exercise.<br />
Example 1 (Z l actions converging to a Z 2 -action.). Consider a 2-torus T 2 k<br />
given by<br />
a Riemannian product S 1 ×S 1 where each of the factors has length k. Put l = k 2 +1<br />
and let Z/lZ act on T 2 k by ( e 2πi<br />
k 2 k2πi )<br />
+1 , e k 2 +1 . In the equivariant Gromov–Hausdorff<br />
topology the action will converge (for k → ∞) to the standard Z 2 action on R 2 .<br />
One can, of course, define a similar sequence of actions on S 3 × S 3 but in this<br />
case the actions will not be uniformly cocompact.<br />
□
8 VITALI KAPOVITCH AND BURKHARD WILKING<br />
Remark 1.9. a) The example shows that it is difficult to relate an open subgroup<br />
in the limit to corresponding subgroups in the sequence.<br />
b) If g1, i . . . , gh i i<br />
is a generator system of some subgroup Γ ′ i ⊆ Γ i, then one can<br />
consider – after passing to a subsequence – a different limit construction to<br />
get a subgroup G ′ ⊂ G: For some fixed R consider all words w in g1, i . . . , gh i i<br />
with the property w ⋆ p i ∈ B R (˜p i ). This defines a subset in the Cayley<br />
graph of Γ ′ i . Consider now all elements in Γ′ i represented by words in the<br />
identity component of this subset. Passing to the limit gives a closed subset<br />
S R ⊂ G of the limit group and one can now take the limit of S R for R → ∞.<br />
This sort of word limit group can be much smaller than the regular limit<br />
group as the above examples shows. Although it depends on the choice<br />
of the generator system, there are occasions where this limit behaves more<br />
natural than the usual. However, this idea is used only indirectly in the<br />
paper.<br />
1.4. Notations and conventions. Throughout the paper we will use the following<br />
conventions.<br />
• As already mentioned − ∫ S f dµ stands for ∫<br />
1<br />
vol(S) S f dµ.<br />
• Mx(f)(x) denotes the maximum function of f evaluated at x, see <strong>Lemma</strong> 1.4<br />
for the definition.<br />
• A map σ : X → Y between inner metric spaces is called a submetry if<br />
σ(B r (p)) = B r (σ(p)) for all r > 0 and p ∈ X.<br />
• If S ⊂ F is a subset of a group, then 〈S〉 denotes the subgroup generated<br />
by S.<br />
• If g 1 and g 2 are elements in a group, [g 1 , g 2 ] := g 1 g 2 g1 −1 g−1 2 denotes the<br />
commutator. For subgroups F 1 , F 2 we put [F 1 , F 2 ] := 〈{[f 1 , f 2 ] | f i ∈ F i }〉.<br />
• Generators b 1 , . . . , b n ∈ F of a group are called a nilpotent basis if [b i , b j ] ∈<br />
〈b 1 , . . . , b i−1 〉 for i < j.<br />
• N ⊳ F means: N is a normal subgroup of F.<br />
• For a Riemannian manifold (M, g) and λ > 0 we let λM denote the Riemannian<br />
manifold (M, λ 2 g).<br />
• For a limit space Y a tangent cone C p Y at p is some Gromov–Hausdorff<br />
limit of (λ i Y, p) for some λ i → ∞. Tangent cones are not always cones and<br />
are not necessarily unique.<br />
• For a limit space Y of manifolds with lower Ricci curvature bound a regular<br />
point p ∈ Y is a point all of whose tangent cones at p are given by R kp . By<br />
a result of Cheeger and Colding [CC97] these points have full measure and<br />
are thus dense.<br />
• For two sequences of pointed metric spaces (X i , p i ), (Y i , q i ) and maps<br />
f i : X i → Y i we use the notation f i : [X i , p i ] → [Y i , q i ] to indicate that<br />
f i converges in some sense to a map from the pointed Gromov–Hausdorff<br />
limit of (X i , p i ) to the pointed Gromov–Hausdorff limit of (Y i , q i ). It usually<br />
does not mean that f i (p i ) = q i . However, if this is the case we write<br />
f i : (X i , p i ) → (Y i , q i ).<br />
• If a group G acts on a metric space we say g ∈ G displaces p ∈ X by r if<br />
d(p, gp) = r. We will denote the orbit of p by G ⋆ p.<br />
• Gromov’s short generator system (or short basis for short) of a fundamental<br />
group is defined at the beginning of section 1.
STRUCTURE OF FUNDAMENTAL GROUPS 9<br />
2. Finite generation of fundamental groups<br />
<strong>Lemma</strong> 2.1 (Product <strong>Lemma</strong>). Let M i be a sequence of manifolds with Ric Mi ><br />
−ε i → 0 satisfying<br />
• B ri (p i ) is compact for all i with r i → ∞ and p i ∈ M i ,<br />
• for every i and j = 1, . . . , k there are harmonic functions b i j : B r i<br />
(p i ) → R<br />
which are L-Lipschitz and fulfill<br />
∫ k∑<br />
k∑<br />
− | < ∇b i j, ∇b i l > −δ jl | + ‖Hess b i<br />
j<br />
‖ 2 dµ → 0 for all R > 0.<br />
B R (p i)<br />
j,l=1<br />
j=1<br />
Then (B ri (p i ), p i ) subconverges in the pointed Gromov–Hausdorff topology to a metric<br />
product (R k ×X, p ∞ ) for some metric space X. Moreover, (b i 1, . . . , b i k ) converges<br />
to the projection onto the Euclidean factor.<br />
The lemma remains true if one just has a uniform lower Ricci curvature bound<br />
and one can also prove a local version of the lemma if r i = R is a fixed number.<br />
However, the above version suffices for our purposes.<br />
Proof. The main problem is to prove this in the case of k = 1. Put b i = b i 1.<br />
After passing to a subsequence we may assume that (B ri (p i ), p i ) converges to some<br />
limit space (Y, p ∞ ). We also may assume that b i converges to an L-Lipschitz map<br />
b ∞ : Y → R.<br />
Step 1. b ∞ is 1-Lipschitz.<br />
This is essentially immediate from the segment inequality. Let x, y ∈ Y be<br />
arbitrary. Choose R so large that x, y ∈ B R/4 (p ∞ ) and let x i , y i ∈ B R/2 (p i ) be<br />
sequences converging to x and y.<br />
For a fixed δ
10 VITALI KAPOVITCH AND BURKHARD WILKING<br />
<strong>Lemma</strong> 2.2. Suppose a sequence of complete inner metric spaces (Y i , p i ) converges<br />
to a locally compact metric space (Y ∞ , p ∞ ). Suppose G i acts isometrically on Y i and<br />
that the action of G i converges to an action of a closed subgroup G ∞ ⊂ Iso(Y ∞ ).<br />
Let G i (ε) denote the subgroup generated by those elements that displace p i by at<br />
most ε, i ∈ N ∪ {∞}. Suppose G ∞ (ε) = G ∞ ( a+b<br />
2<br />
Then there is some sequence ε i<br />
(a + ε i , b − ε i ).<br />
) for all ε ∈ (a, b) and 0 ≤ a < b.<br />
) for all ε ∈<br />
→ 0 such that G i (ε) = G i ( a+b<br />
2<br />
Proof. Suppose on the contrary we can find g i ∈ G i (ε 2 ) \ G i (ε 1 ) for fixed ε 1 < ε 2 ∈<br />
(a, b). Without loss of generality d(p i , g i p i ) ≤ ε 2 .<br />
Because of g i ∉ G i (ε 1 ) it follows that for any finite sequence of points p i =<br />
x 1 , . . . , x h = g i p i ∈ G i ⋆ p i there is one j with d(x j , x j+1 ) ≥ ε 1 . Clearly this<br />
property carries over to the limit and implies that g ∞ ∈ G ∞ (ε 2 ) is not contained<br />
in G((ε 1 + a)/2) – a contradiction.<br />
□<br />
<strong>Lemma</strong> 2.3. Suppose (Mi n, q i) converges to (R k × K, q ∞ ) where Ric Mi ≥ −1/i<br />
and K is compact. Assume the action of π 1 (M i ) on the universal cover ( ˜M i , ˜q i )<br />
converges to a limit action of a group G on some limit space (Y, ˜q ∞ ).<br />
Then G(r) = G(r ′ ) for all r, r ′ > 2 diam(K).<br />
Proof. Since Y/G is isometric to R k ×K, it follows that there is a submetry σ : Y →<br />
R k . It is immediate from the splitting theorem that this submetry has to be linear,<br />
that is, for any geodesic c in Y the curve σ◦c is affine linear. Hence we get a splitting<br />
Y = R k × Z such that G acts trivially on R k and on Z with compact quotient K.<br />
We may think of ˜q ∞ as a point in Z. For g ∈ G consider a mid point x ∈ Z of<br />
˜q ∞ and g˜q ∞ . Because Z/G = K we can find g 2 ∈ G with d(g 2˜q ∞ , x) ≤ diam(K).<br />
Clearly<br />
d(˜q ∞ , g 2˜q ∞ ) ≤ 1 2 d(˜q ∞, g˜q ∞ ) + diam(K)<br />
d(˜q ∞ , g −1<br />
2 g˜q ∞) = d(g 2˜q ∞ , g˜q ∞ ) ≤ 1 2 d(˜q ∞, g˜q ∞ ) + diam(K).<br />
This proves G(r) ⊂ G ( r/2 + diam(K) ) and the lemma follows.<br />
<strong>Lemma</strong> 2.4 (Gap <strong>Lemma</strong>). Suppose we have a sequence of manifolds (M i , p i ) with<br />
a lower Ricci curvature bound converging to some limit space (X, p ∞ ) and suppose<br />
that the limit point p ∞ is regular. Then there is a sequence ε i → 0 and a number<br />
δ > 0 such that the following holds. If γ 1 , . . . , γ li is a short basis of π 1 (M i , p i ) then<br />
either |γ j | ≥ δ or |γ j | < ε i .<br />
Moreover, if the action of π 1 (M i ) on the universal cover ( ˜M i , ˜p i ) converges to<br />
an action of the limit group G on (Y, ˜p ∞ ), then the orbit G ⋆ ˜p ∞ is locally path<br />
connected.<br />
Proof. That the orbit is locally path connected is a consequence of the following<br />
Claim. There is a δ > 0 such that for all points x ∈ G ⋆ ˜p ∞ with d(˜p ∞ , p) < δ we<br />
can find y ∈ G ⋆ ˜p ∞ with max{d(˜p ∞ , y), d(y, x)} ≤ 0.51 · d(˜p ∞ , x).<br />
To prove the claim we argue by contradiction and assume that for some δ h > 0<br />
converging to 0 we can find g h ∈ G with d(g h ˜p ∞ , ˜p ∞ ) = δ h such that for all a ∈ G<br />
with d(a˜p ∞ , ˜p ∞ ) ≤ 0.51δ h we have d(a˜p ∞ , g˜p ∞ ) > 0.51δ h .<br />
After passing to a subsequence we can assume that ( 1<br />
δ h<br />
Y, ˜p ∞ ) converges to a<br />
tangent cone (C p∞ Y, o) and that the action of G on 1<br />
δ h<br />
Y converges to an action of<br />
□
STRUCTURE OF FUNDAMENTAL GROUPS 11<br />
some group K on C p∞ Y . Let g ∞ ∈ K be the limit of g h . Clearly d(o, g ∞ o) = 1 and<br />
for all a ∈ K with d(o, ao) ≤ 0.51 we have d(go, ao) ≥ 0.51. In particular the orbit<br />
K ⋆ o is not convex.<br />
Because X = Y/G the quotient C˜p∞ Y/K is isometric to lim h→∞ ( 1<br />
ε h<br />
X, p ∞ ) which<br />
by assumption is isometric to some Euclidean space R k .<br />
As in the proof of <strong>Lemma</strong> 2.3 it follows that C˜p∞ Y is isometric to R k × Z and<br />
the orbits of the action of K are given by v × Z where v ∈ R k . In particular, the K<br />
orbits are convex – a contradiction.<br />
Clearly, the claim implies that G ⋆ ˜p ∞ ∩ B r (˜p ∞ ) is path connected for r < δ. In<br />
fact, it is now easy to construct a Hölder continuous path from ˜p ∞ to any point in<br />
G ⋆ ˜p ∞ ∩ B r (˜p ∞ ).<br />
Of course, the claim also implies G(ε) = G(δ) for all ε ∈ (0, δ]. Therefore the<br />
first part of <strong>Lemma</strong> 2.4 follows from <strong>Lemma</strong> 2.2.<br />
□<br />
Theorem 3 will follow from the following slightly more general result.<br />
Theorem 2.5. Given n and R there is a constant C such that the following holds.<br />
Suppose (M, g) is a manifold, p ∈ M, B 2R (p) is compact and Ric > −(n − 1) on<br />
B 2R (p). Furthermore, we assume that π 1 (M, g) is generated by loops of length ≤ R.<br />
Then π 1 (M, p) can be generated by C loops of length ≤ R.<br />
Moreover, there is a point q ∈ B R/2 (p) such that any Gromov short generator<br />
system of π 1 (M, q) has at most C elements.<br />
Proof. It is clear that it suffices to show that for a point q ∈ B R/2 (p) the number<br />
of elements in a Gromov short generator system of π 1 (M, q) is bounded above by<br />
some a priori constant.<br />
We consider a Gromov short generator system γ 1 , . . . , γ k of π 1 (M, q). With<br />
|γ i | ≤ |γ i+1 | for some q ∈ B R/2 (p).<br />
Clearly |γ i | ≤ 2R and it easily follows from Bishop–Gromov relative volume<br />
comparison that there are effective a priori bounds (depending only on n, R and r)<br />
for the number of short generators with length ≥ r.<br />
We will argue by contradiction. It will be convenient to modify the assumption<br />
on π 1 being boundedly generated. We call (M i , g i ) a contradicting sequence if the<br />
following holds<br />
• B 3 (p i ) is compact and Ric > −(n − 1) on B 3 (p i ).<br />
• For all q i ∈ B 1 (p i ) the number of short generators of π 1 (M i , q i ) of length<br />
≤ 4 is larger than 2 i .<br />
Clearly it suffices to rule out the existence of a contradicting sequence. We may<br />
assume that (B 3 (p i ), p i ) converges to some limit space (X, p ∞ ). We put<br />
dim(X) = max { k | there is a regular p ∈ B 1/4 (p ∞ ) with C x X ∼ = R k} .<br />
We argue by reverse induction on dim(X). We we start our induction at dim(X) ≥<br />
n + 1. It is well known that this can not happen so there is nothing to prove. The<br />
induction step is subdivided in two substeps.<br />
Step 1. For any contradicting sequence (M i , p i ) converging to (X, p ∞ ) there is a<br />
new contradicting sequence converging to (R dim(X) , 0).<br />
Choose q i ∈ B 1/4 (p i ) converging to some point q ∞ ∈ B 1/4 (p ∞ ) with C q∞ X =
12 VITALI KAPOVITCH AND BURKHARD WILKING<br />
R dim(X) . After passing to a subsequence we can assume that for all x i ∈ B 1/4 (q i )<br />
the number of short generators of π 1 (M i , x i ) of length ≤ 1 is at least 3 i .<br />
Since the number of short generators of length ∈ [ε, 4] is bounded by some a priori<br />
constant, we find a sequence λ i → ∞ very slowly such that for every x ∈ B 1/λi (q i )<br />
the number of short generators of π 1 (M i , x) of length ≤ 4/λ i is at least 2 i and<br />
(λ i M i , q i ) converges to C q∞ X = R dim(X) . Replacing M i by λ i M i and p i and q i<br />
gives a new contradicting sequence, as claimed.<br />
Step 2. If there is a contradicting sequence converging to R k , then we can find a<br />
contradicting sequence converging to a space whose dimension is larger than k.<br />
Let (M i , p i ) be a contradicting sequence converging to (R k , 0). We may assume<br />
without loss of generality that for some r i → ∞ and ε i → 0 the Ricci curvature on<br />
B ri (p i ) is bounded below by −ε i and that B ri (p i ) is compact. In fact, one can run<br />
through the arguments of the first step, to see that, after passing to a subsequence,<br />
a rescaling by λ i → ∞ (very slowly) is always possible.<br />
By Theorem 1.3 we can find a harmonic map (b i 1, . . . , b i k ): B 1(q i ) → R k with<br />
and<br />
∫<br />
−<br />
B 1(q i)<br />
j,l=1<br />
k∑<br />
|〈∇b i l, ∇b i j〉 − δ lj | + ‖Hess(b i l)‖ 2 = ε i → 0<br />
|∇b i j| ≤ C(n).<br />
By the weak (1,1) inequality (<strong>Lemma</strong> 1.4) we can find z i ∈ B 1/2 (q i ) with<br />
∫<br />
−<br />
B r(z i)<br />
j,l=1<br />
k∑<br />
|〈∇b i l, ∇b i j〉 − δ lj | + ‖Hess(b i l)‖ 2 ≤ Cε i → 0<br />
for all r ≤ 1/4. By the Product <strong>Lemma</strong> (2.1), for any sequence µ i → ∞ the spaces<br />
(µ i B r (z i ), z i ) subconverge to a metric product (R k × Z, z ∞ ) for some Z depending<br />
on the rescaling.<br />
From <strong>Lemma</strong>s 2.2 and 2.3 we deduce that there is a sequence δ i → 0 such that<br />
for all z i ∈ B 2 (p i ) the short generator system of π 1 (M i , z i ) does not contain any<br />
elements with length in [δ i , 4].<br />
Choose r i ≤ 1 maximal with the property that there is y i ∈ B ri (z i ) such that<br />
the short generator system of π 1 (M i , y i ) contains one generator of length r i . We<br />
have seen above that r i ≤ δ i → 0<br />
Put N i = 1 r i<br />
M i . By construction, π 1 (N i , y i ) still has 2 i short generators of<br />
length ≤ 1 for all y i ∈ B 1 (z i ) ⊂ N i , and there is one with length 1 for a suitable<br />
y i . By the Product <strong>Lemma</strong> (2.1), (N i , z i ) subconverges to a product (R k × Z, z ∞ ).<br />
<strong>Lemma</strong>s 2.2 and 2.3 imply that Z can not be a point and the claim is proved. □<br />
Finally, let us mention that there is a measured version of Theorem 2.5.<br />
Theorem 2.6. For any n > 1 for any 1 > ε > 0 there exists C(n, ε) such that<br />
if Ric(M n ) ≥ −(n − 1) on B 3 (p) with B 3 (p) compact. Then there is a subset<br />
B 1 (p) ′ ⊂ B 1 (p) with vol B 1 (p) ′ ≥ (1 − ε) vol B 1 (p) and for any q ∈ B 1 (p) ′ the short<br />
basis of π 1 (M, q) has at most C(n, ε) elements of length ≤ 1.<br />
Since we do not have any applications of this theorem, we omit its proof.
STRUCTURE OF FUNDAMENTAL GROUPS 13<br />
3. Maps which are on all scales close to isometries.<br />
For a map f : X → Y between metric spaces we define the distance distortion<br />
on scale r by<br />
(7) dt f r (p, q) = min{r, |d(p, q) − d(f(p), f(q))|}.<br />
Definition 3.1. Let (M i , p 1 i ) and (N i, p 2 i ) be two sequences of Riemannian manifolds<br />
with B i (p j i ) being compact and Ric B i(p j i )<br />
> −C for some C, all i and j = 1, 2.<br />
We say that a sequence of diffeomorphisms f i : M i → N i has the zooming in property<br />
if the following holds:<br />
There exist R 0 > 0, sequences r i → ∞, ε i → 0 and subsets B 2ri (p j i )′ ⊂ B 2ri (p j i )<br />
(j = 1, 2) satisfying<br />
a) vol ( B 1 (q) ∩ B 2ri (p j i )′) ≥ (1 − ε i ) vol(B 1 (q)) for all q ∈ B ri (p j i ).<br />
b) For all p ∈ B ri (p 1 i )′ , all q ∈ B ri (p 2 i )′ and all r ∈ (0, 1] we have<br />
∫<br />
− dt fi<br />
r (x, y)dµ(x)dµ(y) ≤ rε i and<br />
B<br />
∫ r(p)×B r(p)<br />
− dt f −1<br />
i<br />
r (x, y)dµ(x)dµ(y) ≤ rε i .<br />
B r(q)×B r(q)<br />
c) There are subsets S j i ⊂ B 1(p j i ) with vol(Sj i ) ≥ 1 2 vol( B 1 (p j i )) (j = 1, 2) and<br />
f(S 1 i ) ⊂ B R 0<br />
(p 2 i ) and f −1 (S 2 i ) ⊂ B R 0<br />
(p 1 i ).<br />
We will call elements of B 2ri (p 1 i )′ good points and sometimes use the convention<br />
B r (q) ′ := B 2ri (p j i )′ ∩ B r (q) for all B r (q) ⊂ B 2ri (p j i ).<br />
In the applications we will always have N i = M i . However, in some instances<br />
d(p 1 i , p2 i ) → ∞. That is why it might be helpful to also think about maps between<br />
two unrelated pointed manifolds. If the choice of the base points is not clear we<br />
will say that f i : [M i , p 1 i ] → [N i, p 2 i ] has the zooming in property. Notice that we<br />
do not require f i (p 1 i ) = p2 i . However, property c) ensures that the base points are<br />
respected in a weaker sense.<br />
<strong>Lemma</strong> 3.2. Let (M i , p 1 i ), (N i, p 2 i ) and f i be as above. Then, after passing to a<br />
subsequence, (M i , p 1 i ) G−H<br />
−→ (X 1, p 1 ∞), (N i , p 2 i ) G−H<br />
−→ (X 2, p 2 ∞) and f i converges in<br />
i→∞ i→∞<br />
the weakly measured sense to a measure preserving isometry f ∞ : X 1 → X 2 , that is<br />
for each r > 0 there is a sequence δ i → 0 and subsets S i ⊂ B r (p 1 i ) satisfying<br />
• vol(S i ) ≥ (1 − δ i ) vol(B r (p 1 i )) and<br />
• f i|Si is pointwise close to f ∞|Br(p ∞).<br />
Moreover, f −1<br />
i<br />
converges in this sense to f −1<br />
∞ .<br />
First we claim that the following holds.<br />
Sublemma 3.3. There exists C 1 (n) such that for any good point x ∈ B ri (p 1 i )′ and<br />
any r ≤ 1 there is a subset B r (x) ′′ ⊂ B r (x) with<br />
vol B r (x) ′′ ≥ (1 − C 1 ε i ) vol B r (x) and f i (B r (x) ′′ ) ⊂ B 2r (f i (x)).<br />
Proof. The proof proceeds by induction on the size of r as follows. It is clear that<br />
at good points x ∈ B ri (p 1 i )′ the differential of f i has a bilipschitz constant e Cεi<br />
with some universal C provided ε i < 1/2. Therefore it is clear that the statement
14 VITALI KAPOVITCH AND BURKHARD WILKING<br />
holds for all small r. Hence it suffices to prove that if it holds for r ≤ 1/10, then it<br />
holds for 10r. By assumption<br />
∫<br />
−<br />
dt fi<br />
10r (p, q)dµ(p)dµ(q) ≤ 10rε i.<br />
B 10r(x)×B 10r(x)<br />
Furthermore, as long as C 1 ε i ≤ 1/2 our induction assumption implies that there is<br />
a subset S ⊂ B r (x) with vol(S) ≥ 1 2 vol(B r(x)) and f i (S) ⊂ B 2r (f i (x)).<br />
By Bishop–Gromov<br />
∫<br />
− dt fi<br />
10r (p, q)dµ(p)dµ(q) ≤ C 2(n)rε i<br />
S×B 10r(x)<br />
with some universal constant C 2 (n). Therefore there is a subset B 10r (x) ′′ ⊂ B 10r (x)<br />
with vol(B 10r (x)) ′′ ≥ (1 − C 2 (n)ε i /2) vol(B 10r (x)) and<br />
∫<br />
− dt fi<br />
10r (p, q)dµ(p) ≤ 2r for all q ∈ B 10r(x) ′′ .<br />
S<br />
Using that f i (S) ⊂ B 2r (f i (x)) this clearly implies that f i (B 10r (x) ′′ ) ⊂ B 20r (f i (x)).<br />
Thus the sublemma is valid if we put C 1 (n) = C 2 (n)/2 provided we know in addition<br />
that C 1 (n)ε i ≤ 1/2. We can remove the upper bound on ε i by just putting C 1 (n) =<br />
C 2 (n).<br />
□<br />
Of course, a similar inequality holds for f −1<br />
i .<br />
Proof of <strong>Lemma</strong> 3.2. Clearly, we can finish the proof of the lemma by establishing<br />
Claim. Given δ ∈ (0, 1/10) there exists i 0 such that for all x i , y i ∈ B ri (p 1 i )′ with<br />
d(x i , y i ) < 1/2 we have |d(f i (x i ), f i (x i )) − d(x i , y i )| ≤ 10δ for all i ≥ i 0 (δ).<br />
Consider subsets B δ (x i ) ′′ and B δ (y i ) ′′ as in the sublemma. Then vol(B δ (x i ) ′′ ) ≥<br />
1<br />
2 vol(B δ(x i )) for large i and combining with Bishop–Gromov gives<br />
vol(B 1 (x i )) 2 ≤ C 2 (n, δ) vol(B δ (x i ) ′′ ) vol(B δ (y i ) ′′ ).<br />
Thus, ∫<br />
∫<br />
−<br />
dt fi<br />
1 (p, q) ≤ C 2− dt fi<br />
1 (p, q) ≤ C 2ε i .<br />
B δ (x i) ′′ ×B δ (y i) ′′ B 1(x i) 2<br />
Choose i 0 so large that C 2 ε i ≤ δ for i ≥ i 0 . Then for such i we can find x ′ i ∈ B δ(x i ) ′′<br />
and y i ′ ∈ B δ(y i ) ′′ with dt fi<br />
1 (x′ i , y′ i ) ≤ δ. Combining with d(f i(x ′ i ), f i(x i )) ≤ 2δ and<br />
d(f i (y i ′), f i(y i )) ≤ 2δ we deduce dt fi<br />
1 (x i, y i ) ≤ 7δ as claimed. □<br />
<strong>Lemma</strong> 3.4. Consider (M i , p 1 i ), (N i, p 2 i ), (P i, p 3 i ) like above and two sequences of<br />
diffeomorphisms f i : M i → N i and g i : N i → P i with the zooming in property. Then<br />
f i ◦ g i also has the zooming in property.<br />
Proof. By Sublemma 3.3 there is an i 0 such that for all q i ∈ B ri (p) and r ≤ 1/2<br />
there is a subset B r (q i ) ′′ ⊂ B r (q i ) ′ with vol(B r (q i ) ′′ ) ≥ (1−C 1 ε i ) and f i (B r (q i ) ′′ ) ⊂<br />
B 2r (q i ). Thus,<br />
∫<br />
∫<br />
− dt fi◦gi<br />
r (a, b) ≤ C 2 ε i r + e 2nCεi − dt gi<br />
r (f i (a), f i (b))<br />
B r(q) 2 (B r(q) ′′ )<br />
∫<br />
2<br />
≤ C 2 ε i r + C 3 −<br />
r (x, y) ≤ C 4 ε i ,<br />
B 2r(f i(q)) 2 dt gi
STRUCTURE OF FUNDAMENTAL GROUPS 15<br />
where we used that the differential of f i at q ∈ B r (q i ) ′ has a bilipschitz constant<br />
e Cεi and that the ratio of vol(B 2r (f i (q))) and vol(B r (q)) is for large i bounded by<br />
a universal constant.<br />
We are left with the case of r ∈ [1/2, 1]. By <strong>Lemma</strong> 3.2, f i and g i converge in the<br />
weakly measured sense to isometries. Clearly this implies that f i ◦ g i also converges<br />
in the weakly measured sense to an isometry. And this implies that the distortion<br />
function dt fi◦gi<br />
1 has averaged L 1 norm ≤ δ i on every unit ball in B ρi (p i ) for some<br />
ρ i → ∞ and δ i → 0.<br />
□<br />
The next lemma explains the notion zooming in property.<br />
<strong>Lemma</strong> 3.5. Let (M i , p 1 i ), (N i, p 2 i ) and f i be as above. Then there is a ρ i → ∞,<br />
δ i → 0 and Ti<br />
1 ⊂ B ρi (p 1 i ) such that the following holds<br />
• vol(B 1 (q) ∩ Ti 1) ≥ (1 − δ i) vol(B 1 (q)) for all q ∈ B ρi/2(p 1 i ).<br />
• For any sequence of real numbers λ i → ∞ and any sequence q i ∈ Ti<br />
1<br />
f i : (λ i M i , q i ) → (λ i N i , f i (q i ))<br />
has the zooming in property. We say that f i is good on all scales at q i .<br />
Proof. Let G j i = B 2r i<br />
(p j i )′ and B j i = B 2r i<br />
(p i ) \ B 2ri (p j i )′ . After adjusting r i → ∞<br />
and ε i → 0 we may assume vol(B j i ) ≤ ε i vol(B 1 (q)) for all q ∈ B 2ri (p j i ). Let χ ij<br />
be the characteristic function of B j i . By the weak 1-1 inequality there exists a<br />
universal C such that the set<br />
H j i := { x ∈ B ri/2(p j i ) | Mx(χ ij) ≥ √ }<br />
ε i<br />
satisfies<br />
vol(H j i ) ≤ C√ ε i vol(B 1 (q))<br />
for all q ∈ B 2ri (p j i ). We put T i 1 := ( ) ( )<br />
B ri/2(p 1 i )\H1 i ∩ f<br />
−1<br />
i Bri/2(p 2 i )\H2 i and<br />
Ti<br />
2 := f i (Ti 1). Using <strong>Lemma</strong> 3.2 we can find ρ i → ∞ and δ i → 0 such that<br />
vol ( B ρi (p j i ) \ T j )<br />
i ≤ δi vol(B 1 (q)) for all q ∈ B ri (p j i ), j = 1, 2.<br />
By definition of T j i<br />
vol(B r (q) ∩ G j i )<br />
vol(B r (q))<br />
≥ 1 − √ ε i for all q ∈ T j i and all r ≤ 1, j = 1, 2.<br />
Let dt λif<br />
r<br />
denote the distortion on scale r of f i : λ i M i → λ i N i . Clearly<br />
dt λif<br />
r (p, q) = λ i dt fi<br />
r/λ i<br />
(p, q).<br />
Thus for all λ i → ∞ and all q i ∈ T j i<br />
the zooming in property.<br />
the map f i : (λ i M i , q i ) → (λ i N i , f i (q i )) has<br />
□<br />
Proposition 3.6 (First main example). Let α > 1. Consider a sequence of manifolds<br />
(M i , p i ) with a fixed lower Ricci curvature bound and a sequence of time<br />
dependent vector fields Xi<br />
t (piecewise constant in time) with compact support. Let<br />
c i : [0, 1] → B ri (p i ) be an integral curve of Xi t with c i(0) = p i
16 VITALI KAPOVITCH AND BURKHARD WILKING<br />
Assume that Xi<br />
t is divergence free on B r i+100(p i ) and that B ri+100(p i ) is compact.<br />
Put<br />
u s,i (x) := ( Mx ‖∇·Xi s ‖ α) 1/α<br />
and suppose<br />
∫ 1<br />
0<br />
u t,i (c i (t)) = ε i → 0.<br />
Let f i = φ i1 be the flow of X t i evaluated at time 1. Then for all λ i → ∞<br />
f i : (λ i M i , c(0)) → (λ i M i , c(1))<br />
has the zooming in property. Moreover, for any lift ˜fi : ˜Mi → ˜M i of f i to the<br />
universal cover ˜Mi of M i and for any lift ˜p i ∈ ˜M of c(0) = p i the sequence<br />
˜f i : (λ i ˜Mi , ˜p i ) → (λ i ˜Mi , ˜f i (˜p i )) has the zooming in property as well.<br />
The proposition remains valid if the assumption on Xi<br />
t being divergence free<br />
is removed. However, the proof is easier in this case and we do not have any<br />
applications of the more general case. We will need the following<br />
<strong>Lemma</strong> 3.7. There exists (explicit) C = C(n) such that the following holds. Suppose<br />
(M n , g) has Ric ≥ −1 and X t is a vector field with compact support, which<br />
depends on time (piecewise constant). Let c(t) be the integral curve of X t with<br />
c(0) = p 0 ∈ M and assume that X t is divergence free on B 10 (c(t)) for all t ∈ [0, 1].<br />
Let φ t be the flow of X t . Define the distortion function dt r (t)(p, q) of the flow<br />
on scale r by the formula<br />
{<br />
}<br />
dt r (t)(p, q) := min r, max ∣ d(p, q) − d(φ τ (p), φ τ (q))| .<br />
0≤τ≤t<br />
Put ε := ∫ 1<br />
0 Mx(‖∇·X t ‖)(c(t)) dt. Then for any r ≤ 1/10 we have<br />
∫<br />
−<br />
dt r (1)(p, q) dµ(p)dµ(q) ≤ Cr · ε<br />
B r(p 0)×B r(p 0)<br />
and there exists B r (p 0 ) ′ ⊂ B r (p 0 ) such that<br />
vol(B r(p 0) ′ )<br />
vol(B r(p 0))<br />
≥ (1 − Cε) and φ t (B r (p 0 ) ′ ) ⊂ B 2r (c(t)) for all t ∈ [0, 1].<br />
Proof. We prove the statement for a constant in time vector field X t . The general<br />
case is completely analogous except for additional notational problems.<br />
Notice that all estimates are trivial if ε ≥ 2 C<br />
. Therefore it suffices to prove the<br />
statement with a universal constant C(n) for all ε ≤ ε 0 . We put ε 0 = 1/2C and<br />
determine C in the process. We again proceed by induction on the size of r.<br />
Notice that the differential of φ s at c(0) is bilipschitz with bilipschitz constant<br />
e R s<br />
0 ‖∇·X‖(c(t))dt ≤ 1 + 2ε. Thus the <strong>Lemma</strong> holds for very small r.<br />
Suppose the result holds for some r/10 ≤ 1/100. It suffices to prove that it<br />
then holds for r. By induction assumption we know that for any t there exists<br />
B r/10 (c(t)) ′ ⊂ B r/10 (c(t)) such that for any s ∈ [−t, 1 − t] we have<br />
and<br />
vol(B r/10 (c(t)) ′ ) ≥ (1 − Cε) vol(B r/10 (c(t))) ≥ 1 2 vol(B r/10(c(t)))<br />
φ s (B r/10 (c(t)) ′ ) ⊂ B r/5 (c(t + s)),
STRUCTURE OF FUNDAMENTAL GROUPS 17<br />
where we used ε ≤ 1<br />
2C<br />
in the inequality. This easily implies that vol(B r/10(c(t)))<br />
are comparable for all t. More precisely, for any t 1 , t 2 ∈ [0, 1] we have that<br />
(8)<br />
1<br />
C 0<br />
vol B r/10 (c(t 1 )) ≤ vol B r/10 (c(t 2 )) ≤ C 0 vol B r/10 (c(t 1 ))<br />
with a computable universal C 0 = C 0 (n). Put<br />
∫<br />
h(s) = −<br />
dt r (s)(p, q) dµ(p) dµ(q),<br />
B r/10 (p 0) ′ ×B r(c(0))<br />
U := {(p, q) ∈ B r/10 (p 0 ) ′ × B r (c(0)) | dt r (s)(p, q) < r},<br />
φ s (U) := {(φ s (p), φ s (q)) | (p, q) ∈ U},<br />
and dt ′ dt<br />
r(s)(p, q) := lim sup r(s+h)(p,q)<br />
h↘0 h<br />
. Since dt r is monotonously increasing<br />
and bounded by r, we deduce that if dt r (s)(p, q) = r, then dt ′ r(s)(p, q) = 0.<br />
Since φ s is measure preserving, it follows<br />
∫<br />
h ′ d<br />
(s) ≤ −<br />
dt |t=0 dt′ r(0)(p, q)<br />
φ s(U)<br />
∫<br />
≤<br />
4 vol(B3r(c(s))2<br />
vol(B r/10 (c(0)))<br />
− dt ′ r(0)(p, q),<br />
2<br />
B 3r(c(s)) 2<br />
where we used that φ s (B r/10 (p 0 ) ′ ) 2 ⊂ φ s (U) ⊂ B 3r (c(s)) 2 . If p is not in the cut<br />
locus of q and γ pq : [0, 1] → M is a minimal geodesic between p and q then<br />
dt ′ r(0)(p, q) ≤ d(p, q)<br />
∫ 1<br />
0<br />
‖∇·X‖(γ pq (t)) dt.<br />
Combining the last two inequalities with the segment inequality we deduce<br />
h ′ (s) ≤<br />
∫<br />
C 1 (n)r− ‖∇·X‖<br />
B 6r(c(s))<br />
≤ C 1 (n)r Mx ‖∇·X‖(c(s))<br />
and thus h(1) ≤ C 1 (n)rε. Note that the choice of the constant C 1 (n) can be made<br />
explicit and independent of the induction assumption.<br />
This in turn implies that that the subset<br />
{<br />
∫<br />
}<br />
B r (p 0 ) ′ := p ∈ B r (p 0 ) ∣ − dt r (1)(p, q) dµ(q) ≤ r/2<br />
B r/10 (p 0) ′<br />
satisfies<br />
(9)<br />
It is elementary to check that<br />
vol(B r (p 0 ) ′ ) ≥ (1 − 2C 1 (n)ε) vol(B r (p 0 )).<br />
φ t (B r (p 0 ) ′ ) ⊂ B 2r (c(t)) for all t ∈ [0, 1].<br />
Then arguing as before we estimate that<br />
∫<br />
−<br />
dt r (s)(p, q)dµ(p)dµ(q) ≤ C 2 (n) · r · ε.<br />
B r(c(t)) ′ ×B r(c(t))
18 VITALI KAPOVITCH AND BURKHARD WILKING<br />
Using the definition of dt r (s) and the volume estimate (9) this gives<br />
∫<br />
− dt r (s)(p, q) dµ(p) dµ(q) ≤ C 2 (n) · r · ε + 2rC 1 ε =: C 3 rε.<br />
B r(c(t)) 2<br />
This completes the induction step with C(n) = C 3 and ε 0 = 1<br />
2C 3<br />
. In order to<br />
remove the restriction ε ≤ ε 0 one can just increase C 3 by the factor 4, as indicated<br />
at the beginning.<br />
□<br />
Proof of Proposition 3.6. Put g t,i (x) = ‖∇·Xi t ‖(x). First notice that by Hölder<br />
∫ 1<br />
0<br />
Mx(g t,i (x))(c i (t)) dt ≤<br />
∫ 1<br />
0<br />
u t,i (c i (t)) dt → 0.<br />
Let λ i → ∞ and put r i = R λ i<br />
, where R > 1 is arbitrary. By <strong>Lemma</strong> 3.7 there is<br />
S i ⊂ B ri (c i (0)) with<br />
• vol(S i ) ≥ (1 − δ i ) vol(B ri (c i (0))) with δ i → 0 and<br />
• φ t (S i ) ⊂ B 2ri (c(t)) for all t.<br />
In the following we assume that i is so large that δ i ≤ 1/2, and r i ≤ 1/100. As<br />
in the proof of <strong>Lemma</strong> 3.7 this easily implies that there is a universal constant<br />
C = C(n) with<br />
and<br />
Using that φ it|Bri (c(0)) ′<br />
vol(B 2ri (c i(t)))<br />
vol(B ri (c i(0))) ≤ C 2<br />
for all t and all i.<br />
is measure preserving, we deduce<br />
∫ 1<br />
− Mx(g t,i )(φ it (p)) dt dµ(p) ≤ C<br />
∫S i 0<br />
≤<br />
by (5)<br />
C<br />
∫ 1<br />
0<br />
∫ 1<br />
0<br />
∫<br />
−<br />
B 2ri (c i(t))<br />
Mx(g t,i )(q) dt dµ(q)<br />
Mx(Mx(g t,i ))(c i (t)) dt<br />
∫ 1<br />
≤ C · C 2 Mx(gt,i) α 1/α (c i (t)) dt<br />
0<br />
∫ 1<br />
= C · C 2 u t,i (c i (t)) dt → 0.<br />
Thus we can find ˜δ i → 0 and a subset B ri (c i (0)) ′′ ⊂ S i with<br />
vol(B ri (c i (0))) − vol(B ri (c i (0)) ′′ ) ≤ ˜δ i min vol(B r<br />
q∈B ri (c i/R(q))<br />
i(0))<br />
∫ 1<br />
0<br />
Mx(g t,i )(φ it (q)) ≤ ˜δ i for all q ∈ B ri (c(0)) ′′ .<br />
Recall that r i = R λ i<br />
with an arbitrary R. By a diagonal sequence argument it is<br />
easy to deduce that after replacing ˜δ i by a another sequence converging slowly to<br />
0 we can keep the above estimates for r i = Ri<br />
λ i<br />
with R i → ∞ sufficiently slowly.<br />
Combining this with <strong>Lemma</strong> 3.7 shows that f i = φ i1 : (λ i M i , c(0)) → (λ i M i , c(1))<br />
has the zooming in property.<br />
Let ˜X i t be a lift of Xt i to the universal covering ˜M i of M i . Consider the integral<br />
curve ˜c i : [0, 1] → ˜M i of ˜Xt i with ˜c(0) = ˜p i . Clearly ˜c i is a lift of c i and by the<br />
0
STRUCTURE OF FUNDAMENTAL GROUPS 19<br />
Covering <strong>Lemma</strong> (1.6) we have<br />
∫ 1<br />
0<br />
Mx(‖∇· ˜Xt i ‖ α ) 1/α (˜c i (t)) dt ≤ C<br />
∫ 1<br />
0<br />
Mx(‖∇·X t i ‖ α ) 1/α (c i (t)) dt<br />
with some universal constant C. Thus ˜φ i1 : (λ i ˜Mi , ˜p i ) → (λ i ˜Mi , φ i1 (˜p i )) has the<br />
zooming in property as well. Any other lift ˜f i of f i is obtained by composing ˜φ i1<br />
with a deck transformation and thus the result carries over to any lift of f i .<br />
□<br />
Proposition 3.8 (Second main example). Let (M i , g i ) be a sequence of manifolds<br />
with Ric > −1/i on B i (p i ) and B i (p i ) compact. Suppose that (M i , p i ) converges to<br />
(R k × Y, p ∞ ).<br />
Then for each v ∈ R k there is a sequence of diffeomorphisms f i : [M i , p i ] →<br />
[M i , p i ] with the zooming in property which converges in the weakly measured sense<br />
to an isometry f ∞ of R k × Y that acts trivially on Y and by w ↦→ w + v on R k .<br />
Moreover, f i is isotopic to the identity and there is a lift ˜f i : [ ˜M i , ˜p i ] → [ ˜M i , ˜p i ] of<br />
f i to the universal cover which has the zooming in property as well.<br />
Proof. Using the splitting R k = Rv ⊕(v) ⊥ and replacing Y by Y ×(v) ⊥ we see that<br />
it suffices to prove the statement for k = 1.<br />
By the work of Cheeger and Colding [CC96] we can find sequences ρ i → ∞ and<br />
ε i → 0 and harmonic functions b i : B 4ρi (p i ) → R such that<br />
|∇b i | ≤ L(n) for all i and<br />
∫<br />
(∣<br />
− ∣|∇bi | − 1 ∣ + ‖Hessbi ‖ ) 2<br />
≤ ε i for any R ∈ [1/4, 4ρ i ].<br />
B 4R (p i)<br />
Let X i be a vector field with compact support with X i = ∇b i on B 3ρi (p i ), and<br />
let φ it denote the flow of X i . Clearly for any t we can find r i → ∞ such that<br />
φ it|Bri (p i) is measure preserving.<br />
Put ψ i := ∣ ∣|∇b i | − 1 ∣ + ‖Hess(b i )‖. We deduce from <strong>Lemma</strong> 1.4 that<br />
∫<br />
− Mx(ψ i ) 2 ≤ C(n, R)ε i<br />
and Cauchy Schwarz gives<br />
∫<br />
−<br />
B 2R (p i)<br />
B R (p i)<br />
Mx(ψ i ) ≤ √ C(n, R)ε i .<br />
Suppose now that t 0 ≤ R<br />
4L(n) . Then φ t(q) ∈ B 3R (p i ) for all q ∈ B<br />
4<br />
R/2 (p i )<br />
and all t ∈ [−t 0 , t 0 ]. Combining that φ t is measure preserving and vol(B R (p i )) ≤<br />
C 3 vol(B R/2 (p i )), we get that<br />
∫ ∫ t0<br />
∫<br />
−<br />
Mx(ψ i )(φ t (p)) ≤ C 3 t 0 − Mx(ψ)<br />
B R/2 (p i)<br />
0<br />
It is now easy to find R i → ∞ and δ i → 0 with<br />
B R (p i)<br />
≤ C 4 (t 0 , R, n) √ ε i .<br />
∫ ∫ t0<br />
− Mx(ψ i )(φ t (p)) ≤ δ i for all R ∈ [1, R i ].<br />
B R (p i) 0
20 VITALI KAPOVITCH AND BURKHARD WILKING<br />
After an adjustment of the sequences δ i → 0 and R i → ∞ we can find B Ri (p i ) ′ ⊂<br />
B Ri (p i ) with<br />
vol(B Ri (p i )) − vol(B Ri (p i ) ′ ) ≤ δ i vol(B 1 (q)) for all q ∈ B Ri (p i ) and<br />
∫ t0<br />
0<br />
Mx(ψ i )(φ t (q)) ≤ δ i for all q ∈ B Ri (p i ) ′ .<br />
Moreover, the displacement of φ it0 is globally bounded by L(n)t 0 and thus,<br />
<strong>Lemma</strong> 3.7 implies that φ it0 has the zooming in property.<br />
As in the proof of the Product <strong>Lemma</strong> (2.1) we see that b i converges to the<br />
projection b ∞ : R × Y → R. In particular b ∞ is a submetry.<br />
For all q i ∈ B Ri (p i ) ′ the length of the integral curve c i (t) = φ it (q) (t ∈ [0, t 0 ]) is<br />
bounded above by t 0 +δ i . Moreover, b i (c(t 0 ))−b i (c i (0)) is bounded below by t 0 −δ i .<br />
This implies that φ ∞t0 satisfies d(p, φ ∞t0 (p)) ≤ t 0 and b ∞ (φ ∞t0 (q)) − b ∞ (p) ≥ t 0 .<br />
Clearly, equality must hold and φ ∞t (q) is a horizontal geodesic with respect to the<br />
submetry b ∞ . This in turn implies that φ ∞t0 respects the decomposition Y × R, it<br />
acts by the identity on the first factor and by translation on the second as claimed.<br />
Let σ : ˜Mi → M i denote the universal cover, ˜p i ∈ ˜M i a lift of p i , and let ˜X i be a<br />
lift of X i . By <strong>Lemma</strong> 1.6 all estimates for X i on B R (p i ) give similar estimates for<br />
˜X i on B R (˜p i ). Thus the flow ˜φ it of ˜Xi has the zooming in property as well. □<br />
4. A rough idea of the proof of the <strong>Margulis</strong> <strong>Lemma</strong>.<br />
The proof of the <strong>Margulis</strong> <strong>Lemma</strong> is somewhat indirect and it might not be easy<br />
for everyone to grasp immediately how things interplay. In this section we want to<br />
give a rough idea of how the arguments would unwrap in a simple but nontrivial<br />
case.<br />
Let p i → ∞ be a sequence of odd primes and let Γ i := Z ⋉ Z pi be the semidirect<br />
product where the homomorphism Z → Aut(Z pi ) maps 1 ∈ Z to the automorphism<br />
ϕ i given by ϕ i (z + p i Z) = 2z + p i Z.<br />
Suppose, contrary to the <strong>Margulis</strong> <strong>Lemma</strong>, we have a sequence of compact n-<br />
manifolds M i with Ric > −1/i and diam(M i ) = 1 and fundamental group Γ i .<br />
A typical problem situation would be that M i converges to a circle and its<br />
universal cover ˜M i converges to R.<br />
We then replace M i by B i = ˜M i /Z pi and in order not to loose information we<br />
endow B i with the deck transformation f i : B i → B i representing a generator of<br />
Γ i /Z pi . Then B i will converge to R as well.<br />
It is part of the rescaling theorem that one can find λ i → ∞ such that the<br />
rescaled sequence λ i B i converges to R × K with K being compact but not equal to<br />
a point. Suppose for illustration that λ i B i converges to R × S 1 and λ i ˜Mi converges<br />
to R 2 and that the action of Z pi converges to a discrete action of Z on R 2 .<br />
The maps f i : λ i B i → λ i B i do not converge, because typically f i would map a<br />
base point x i to some point y i = f i (x i ) with d(x i , y i ) = λ i → ∞ with respect to<br />
the rescaled distance.<br />
But a second statement in the rescaling theorem guarantees that we can find a<br />
sequence of diffeomorphisms g i : [λ i B i , y i ] → [λ i B i , x i ] with the zooming in property.<br />
The composition f new,i := f i ◦ g i : [λ i B i , x i ] → [λ i B i , x i ] also has the zooming<br />
in property and thus converges to an isometry of the limit.
STRUCTURE OF FUNDAMENTAL GROUPS 21<br />
Moreover, a lift ˜f new,i : λ i ˜Mi → λ i ˜Mi of f new,i has the zooming in property, too.<br />
Since g i can be chosen isotopic to the identity, the action of ˜fnew,i on the deck<br />
transformation group Z pi = π 1 (B i ) by conjugation remains unchanged.<br />
On the other hand, the Z pi -action on ˜M i converges to a discrete Z-action on<br />
R 2 and ˜f new,i converges to an isometry ˜f new,∞ of R 2 normalizing the Z-action.<br />
This implies that ˜f new,∞ 2 commutes with the Z-action and it is then easy to get a<br />
contradiction.<br />
Problem. We suspect that for any given n and D only finitely many of the groups<br />
in the above family of groups should occur as fundamental groups of compact n-<br />
manifolds with Ric > −(n − 1) and diameter ≤ D. Note that this is not even<br />
known under the stronger assumption of K > −1 and diam ≤ D.<br />
5. The Rescaling Theorem.<br />
Theorem 5.1 (Rescaling Theorem). Let (M, g i , p i ) be a sequence of n-manifolds<br />
satisfying Ric Bri (p i) > −µ i and B ri (p i ) is compact for some r i → ∞, µ i → 0.<br />
Suppose that (M, g i , p i ) → (R k , 0) for some k < n. Then after passing to a subsequence<br />
we can find a compact metric space K with diam(K) = 10 −n2 , a sequence<br />
of subsets G 1 (p i ) ⊂ B 1 (p i ) with vol(G1(pi))<br />
vol(B → 1 and a sequence λ 1(p i)<br />
i → ∞ such that<br />
the following holds<br />
• For all q i ∈ G 1 (p i ) the isometry type of the limit of any convergent subsequence<br />
of (λ i M i , q i ) is given by the metric product R k × K.<br />
• For all a i , b i ∈ G 1 (p i ) we can find a sequence of diffeomorphisms<br />
f i : [λ i M i , a i ] → [λ i M i , b i ]<br />
with the zooming in property such that f i is isotopic to the identity. Moreover,<br />
for any lift ã i , ˜b i ∈ ˜M i of a i and b i to the universal cover ˜M i we can<br />
find a lift ˜f i of f i such that<br />
˜f i : [λ i ˜Mi , ã i ] → [λ i ˜Mi , ˜b i ]<br />
has the zooming in property as well.<br />
Finally, if π 1 (M i , p i ) is generated by loops of length ≤ R for all i, then we can<br />
find ε i → 0 such that π 1 (M i , q i ) is generated by loops of length ≤ 1+εi<br />
λ i<br />
for all<br />
q i ∈ G 1 (p i ).<br />
Proof. By Theorem 1.3, after passing to a subsequence we can find a harmonic map<br />
b i : B i (p i ) → B i+1 (0) ⊂ R k that gives a 1 2<br />
Gromov–Hausdorff approximation from<br />
i<br />
B i (p i ) to B i (0) ⊂ R k . Put<br />
h i =<br />
k∑<br />
| < ∇b i j, ∇b i l > −δ j,l | + ∑ j<br />
j,l=1<br />
‖Hess b i<br />
j<br />
‖ 2 .<br />
After passing to a subsequence we also have by Theorem 1.3 that<br />
∫<br />
− h i ≤ ε 4 i for all R ∈ [1, i] with ε i → 0.<br />
B R (p i)<br />
Furthermore, we may assume Ric > −ε i on B i (0).<br />
(10) G 4 (p i ) := { x ∈ B 4 (p i ) | Mx h i (x) ≤ ε 2 i<br />
}<br />
.
22 VITALI KAPOVITCH AND BURKHARD WILKING<br />
We will call elements of G 4 (p i ) ”good points” in B 4 (p i ). We put G r (q) = B r (q) ∩<br />
G 4 (p i ) for q ∈ B 2 (p i ) and r ≤ 2. It is immediate from the weak 1-1 inequality<br />
(<strong>Lemma</strong> 1.4) that<br />
vol(G 1(p i))<br />
vol(B 1(p i)) → 1.<br />
By the Product <strong>Lemma</strong> (2.1), for any q i ∈ G 4 (p i ) and any h ≥ 1 we have that<br />
hB 1/h (q i ) is ˜ε i close to a ball in a product R k × Y where ˜ε i → 0 can be chosen<br />
independently of h and q i . Note that the space Y may depend on h and q i .<br />
For any point q i ∈ G 1 (p i ) let ρ i (q i ) be the largest number ρ < 1 such that the<br />
map b i : B ρ (q i ) → R k has distance distortion exactly equal to ρ · 10 −n2 . For any<br />
choice q i ∈ G 1 (p i ) we know that ρ i (q i ) → 0 and by the Product <strong>Lemma</strong> (2.1)<br />
(<br />
1<br />
ρ M )<br />
i(q i) i, q i subconverges to R k × K where K is compact with diam(K) = 10 −n2 .<br />
Again note, that a priori, the constants ρ i (q i ) and the space K depend on the<br />
choice of q i ∈ B 1 (p i ) ′ . We will show that, in fact, both are essentially independent<br />
of the choice of good points in B 1 (p i ).<br />
For each i we define the number ρ i as the supremum of ρ i (q i ) with q i ∈ G 1 (p i )<br />
and put<br />
λ i = 1 ρ i<br />
.<br />
The statement of the theorem will follow easily from the following sublemma.<br />
Sublemma 5.2. There exist constants ¯C 1 , ¯C 2 independent of i such that the following<br />
holds. For any good point p ∈ G 1 (p i ) and any r ≤ 2 there exists B r (p) ′ ⊂ B r (p)<br />
such that<br />
(11) vol B r (p) ′ ≥ (1 − ¯C 1 rε i ) vol B r (p)<br />
and such that for any q ∈ B r (p) ′ there exists p ′ ∈ B Lρi (p) (with L = 9 n ) and a time<br />
dependent (piecewise constant in time) divergence free vector field X t (dependence<br />
on q is suppressed) and its integral curve c q (t): [0, 1] → B 2 (p i ) such that c(0) =<br />
p ′ , c(1) = q and<br />
1<br />
vol(B r(p))<br />
∫B r(p) ′ ∫ 1<br />
0<br />
(<br />
Mx(‖∇·X t ‖ 3/2 ) ) 2/3<br />
(cq (t)) dt dµ(q) ≤ ¯C 2 rε i .<br />
Proof. Despite the fact that L can be chosen as L = 9 n we treat it for now as a<br />
large constant L ≥ 10 which needs to be determined. Throughout the proof we will<br />
denote by C j various constants depending on n. Sometimes these constants will<br />
also depend on L in which case that dependence will always be explicitly stated.<br />
Although we consider a fixed i for the major part of the proof, we take the<br />
liberty of assuming that i is large. This way we can ensure that for any r ≥ ρ i<br />
1<br />
and any good point p ∈ G 1 (p i ) the ball<br />
Lr B 2Lr(p) is by the Product <strong>Lemma</strong> in the<br />
measured Gromov–Hausdorff sense arbitrarily close to a ball in a product R k × K<br />
where by our choice of ρ i , K has diameter ≤ 10 −n2 . This implies, for example, that<br />
we can assume that<br />
(<br />
1<br />
(12)<br />
r1<br />
) k vol(B<br />
2 r 2<br />
≤<br />
r1 (q 1))<br />
vol(B r2 (q ≤ 2( )<br />
r 1 k<br />
2)) r 2<br />
for all q1 , q 2 ∈ B Lr (p), r 1 , r 2 ∈ [r/20, 2Lr].<br />
The sublemma is trivially true for r ≤ Lρ i with X ≡ 0. By induction it suffices<br />
to show that if the sublemma holds for some r ∈ (ρ i , 2 L<br />
] then it holds for Lr.
STRUCTURE OF FUNDAMENTAL GROUPS 23<br />
Let q 1 , . . . q l be a maximal r/2-separated net in B Lr (p). Note that<br />
l⋃<br />
B Lr (p) ⊂ B r/2 (q m ).<br />
m=1<br />
By a standard volume argument we can deduce from (12) that<br />
(13)<br />
∑ l<br />
m=1 vol B r/2(q m )<br />
≤ 3 k+1 ≤ 3 n .<br />
vol B Lr (p)<br />
Fix q m and consider the following vector field<br />
k∑<br />
X(x) = (b i α(p) − b i α(q m ))∇b i α(x).<br />
α=1<br />
Since b i α are Lipschitz with a universal Lipschitz constant by (1), X satisfies<br />
(14) |X(x)| ≤ C(n) · Lr.<br />
Also, by construction, X satisfies<br />
(15) Mx ‖∇·X‖ 2 (p) ≤ C 4 ε 2 i L 2 r 2 .<br />
Note that we get an extra L 2 r 2 factor as compared to (10) because |b i (p)−b i (q m )| ≤<br />
C(n)Lr. By applying (6) we get<br />
(<br />
Mx [Mx(‖∇·X‖ 3/2 )] 4/3) (p) ≤ C(n) Mx(‖∇·X‖ 2 )(p) ≤ (C 5 Lε i r) 2 .<br />
In particular, for R 1 = 2C(n)Lr<br />
∫<br />
− [Mx(‖∇·X‖ 3/2 )] 4/3 (p) ≤ (C 5 Lε i r) 2<br />
B R1 (p)<br />
provided R 1 ≤ 1. However, in the case of R 1 ∈ [1, 4C(n)] the same follows more<br />
directly from our initial assumptions combined with <strong>Lemma</strong> 1.4 for all large i. By<br />
Cauchy inequality the last estimate gives<br />
∫<br />
(<br />
(16) − Mx(‖∇·X‖ 3/2 ) ) 2/3<br />
(p) ≤ C5 Lε i r.<br />
B R1 (p)<br />
Consider the measure preserving flow φ of X on [0, 1]. Because of (14) the flow<br />
lines φ t (q) with q ∈ B r (p) stay in the ball B R1 (p) for t ∈ [0, 1]. We choose a<br />
universal constant C 6 (L) with C 5 L · vol(B 2C(n)Lr(p))<br />
vol(B r/10 (q m)<br />
≤ C 6 (L)<br />
Combining that the flow is measure preserving, inequality (16) and our choice<br />
of C 6 we see that<br />
(17)<br />
∫<br />
1<br />
vol(B r (q 10 m))<br />
B r(q m)<br />
∫ 1<br />
0<br />
(<br />
Mx(‖∇·X‖ 3/2 ) ) 2/3<br />
(φt (x)) dt dµ(x) ≤ C 6 (L)rε i .<br />
By the Product <strong>Lemma</strong> (2.1) applied to the rescaled balls<br />
1<br />
Lr B Lr(p) we know<br />
that 1 Lr B Lr(p) is measured Gromov–Hausdorff close to a unit ball in R k ×K 3 where<br />
diam K 3 ≤ 10 −n2 . Moreover, φ 1 is measured close to a translation by b i (p)−b i (q m )<br />
in R k , see proof of Proposition 3.8. Since we only need to argue for large i, we may
24 VITALI KAPOVITCH AND BURKHARD WILKING<br />
assume that by volume 3/4 of the points in B r/10 (q m ) are mapped by φ 1 to points<br />
in B r/9 (p).<br />
We choose such a point q ∈ B r/10 (q m ). In view of (17) we may assume in<br />
addition that<br />
∫ 1<br />
By <strong>Lemma</strong> 3.7 this implies<br />
0<br />
Mx(‖∇·X‖ 3/2 ) 2/3 (φ t (q)) dt ≤ 4 3 C 6(L)rε i .<br />
∫<br />
(18) − dt r (1)(x, y) dµ(x) dµ(y) ≤ C 7 (L)r · ε i .<br />
B r(q)×B r(q)<br />
Note that by definition, dt r (1)(x, y) ≥ min{r, |d(φ 1 (x), φ 1 (y)−d(x, y)|}. Combining<br />
this inequality with the knowledge that 3/4 of the points in B r/10 (q m ) end up in<br />
B 1/9r (p), implies that we can find a subset B r/2 (q m ) ′ ⊂ B r/2 (q m ) with<br />
(19)<br />
(20)<br />
vol(B r/2 (q m ) ′ ) ≥ (1 − C 8 (L)ε i r) vol(B r/2 (q m )) and<br />
φ 1 (B r/2 (q m ) ′ ) ⊂ B r (p).<br />
Set B r/2 (q m ) ′′ := B r/2 (q m ) ′ ∩ φ −1<br />
1 (B r(p) ′ ). Then we get<br />
φ 1 (B r/2 (q m ) ′′ ) ⊂ B r (p) ′<br />
and<br />
vol(B r/2 (q m ) ′′ ) ≥ vol(B r/2 (q m ) ′ ) − (vol B r (p) − vol B r (p) ′ )<br />
(21)<br />
by (19) and (11)<br />
≥ (1 − C 8 (L)rε i ) vol B r/2 (q m ) − ¯C 1 rε i vol B r (p)<br />
by (12)<br />
≥ (1 − C 8 (L)rε i ) vol B r/2 (q m ) − 2 k+1 ¯C1 rε i vol B r/2 (q m )<br />
≥ (1 − (C 8 (L) + 2 n ¯C1 )rε i ) vol B r/2 (q m ),<br />
where we used that φ 1 is volume-preserving in the first inequality.<br />
Using the induction assumption, we can prolong each integral curve from any<br />
q ∈ B r/2 (q m ) ′′ to a point x ∈ B r (p) ′ by extending it by a previously constructed<br />
integral curve from x to p ′ ∈ B Lρi (p) of a vector field Xold t (which depends on p′ ).<br />
We set Xnew t = 2Xold 2t for 0 ≤ t ≤ 1/2 and Xt new = −2X for 1/2 ≤ t ≤ 1. Let c q (t)<br />
be the integral curve of Xnew t with c q (1) = q and c q (0) = p ′ .<br />
By the induction assumption and (17), we get<br />
(22)<br />
∫B r/2 (q m) ′′ ∫ 1<br />
0<br />
(Mx ‖∇·X t new‖ 3/2 ) 2/3 (c q (t)) dt dµ(q) ≤<br />
≤ C 6 (L)(rε i ) vol B r/2 (q m ) + ¯C 2 (rε i ) vol B r (p)<br />
(12)<br />
≤ C 6 (L)(rε i ) vol B r/2 (q m ) + 2 n ¯C2 (rε i ) vol B r/2 (q m )<br />
= ( C 9 (L) + 2 n ¯C2<br />
)<br />
(rεi ) vol B r/2 (q m ).<br />
Recall that the balls B r/2 (q m ) cover B Lr (p). We put<br />
B Lr (p) ′ := B Lr (p) ∩ ⋃ m<br />
B r/2 (q m ) ′′ .
STRUCTURE OF FUNDAMENTAL GROUPS 25<br />
By construction, for every point in B Lr (p) ′ there exists a vector field X t whose<br />
integral curve connects it to a point in B Lρi (p) satisfying (22). For points covered<br />
by several sets B r/2 (q m ) ′′ we pick any one. Then we have<br />
∫B Lr (p) ′ ∫ 1<br />
0<br />
Mx ( ‖∇·Xnew‖ t ) 3 2<br />
2 3<br />
(c q (t)) dt dµ(q) ≤<br />
≤<br />
(13)<br />
≤<br />
≤<br />
l∑<br />
(C 9 (L) + 2 n ¯C2 )(rε i ) vol B r/2 (q m )<br />
m=1<br />
3 n (C 9 (L) + 2 n ¯C2 )(rε i ) vol B rL (p)<br />
¯C 2 · Lrε i · vol B rL (p).<br />
The last inequality holds if L = 9 n ≥ 2 · 2 n · 3 n and ¯C 2 ≥ 3 n · C 9 (L).<br />
Recall that by (21)<br />
vol B r/2 (q m ) − vol B r/2 (q m ) ′′ ≥ (C 8 (L) + 2 n ¯C1 )rε i vol B r/2 (q m )<br />
for any m and thus<br />
vol B Lr (p) − vol B Lr (p) ′<br />
≤<br />
(13)<br />
≤<br />
≤<br />
l∑<br />
(C 8 (L) + 2 n ¯C1 )rε i vol B r/2 (q m )<br />
m=1<br />
provided that L = 9 n ≥ 2 · 6 n and ¯C 1 ≥ 3 n C 8 (L).<br />
This finishes the proof of Sublemma 5.2.<br />
3 n (C 8 (L) + 2 n ¯C1 )rε i vol(B Lr (p))<br />
¯C 1 Lrε i vol B Lr (p)<br />
Observe that for any two good points x i , y i ∈ G 1 (p i ) we can deduce from Sublemma<br />
5.2 that vol(B 2 (x i ) ′ ∩ B 2 (y i ) ′ ) ≥ vol(B 1/2 (p i )) for all large i.<br />
Then it follows, also by Sublemma 5.2, that there is q ∈ B 2 (x i ) ′ ∩ B 2 (y i ) ′ ,<br />
x ′ i ∈ B Lρ i<br />
(x i ) and y i ′ ∈ B Lρ i<br />
(y i ) such that for the integral curve c connecting x ′ i<br />
with q on the first half of the interval and q with y i ′ on the second half we have for<br />
the corresponding time dependent vector field Xi<br />
t<br />
∫ 1<br />
(<br />
Mx(‖∇·X t i ‖ 3/2 ) ) 2/3<br />
(c(t)) dt ≤ ¯C3 ε i<br />
□<br />
0<br />
with a constant ¯C 3 independent of i.<br />
Let f i := φ i1 be the flow of X t i evaluated at time 1. Since λ i = 1 ρ i<br />
, we can<br />
employ Proposition 3.6 to see that<br />
f i : [λ i M i , x i ] → [λ i M i , y i ]<br />
has the zooming in property. Thus, the Gromov–Hausdorff limits of the two sequences<br />
are isometric.<br />
For any lifts ˜x i and ỹ i of x i and y i to the universal cover we can lifts ˜x ′ i and ỹ′ i<br />
of x ′ i and y′ i in the Lρ i neighbourhoods of ˜x i and ỹ i . Let ˜f i : ˜Mi → ˜M i be the lift<br />
f i with ˜f i (˜x ′ i ) = ỹ′ i . Proposition 3.6 ensures that ˜f i : [λ i ˜Mi , ˜x i ] → [λ i ˜Mi , ỹ i ] has the<br />
zooming in property as well.<br />
It remains to check the last part of the Rescaling Theorem concerning the fundamental<br />
group. Assume that π 1 (M i , p i ) is generated by loops of length ≤ R.
26 VITALI KAPOVITCH AND BURKHARD WILKING<br />
For every good point q ∈ B 1 (p i ) let r i (q) denote the minimal number such that<br />
π 1 (M i , q) can be generated by loops of length ≤ r i (q). Let r i denote the supremum<br />
of r i (q) over all good points q and choose a good point q i with r i (q i ) ≥ 1 2 r i. It<br />
suffices to check that lim sup i→∞ λ i r i ≤ 1. Suppose we can find a subsequence<br />
with λ i r i ≥ 1/4 for all i. By the Product <strong>Lemma</strong> (2.1), the sequence ( 1 r i<br />
M i , q i )<br />
subconverges to (R k × K ′ , q ∞ ) with diam(K ′ ) ≤ 4 · 10 −n2 . Combining <strong>Lemma</strong> 2.2<br />
with <strong>Lemma</strong> 2.3 we see that π 1 ( 1 r i<br />
M i , q i ) can be generated by loops of length<br />
≤ 2 diam(K ′ ) + c i with c i → 0 – a contradiction.<br />
□<br />
6. The Induction Theorem for C-Nilpotency<br />
C-nilpotency of fundamental groups of manifolds with almost nonnegative Ricci<br />
curvature (Corollary 2) will follow from the following technical result.<br />
Theorem 6.1 (Induction Theorem). Suppose (M n i , p i) is a sequence of pointed<br />
n-dimensional Riemannian manifolds (not necessarily complete) satisfying<br />
(1) Ric Mi ≥ −1/i;<br />
(2) B i (p i ) is compact for any i;<br />
(3) There is some R > 0 such that π 1 (B R (p i )) → π 1 (M i ) is surjective for all i;<br />
(4) (M i , p i ) G−H<br />
−→<br />
i→∞ (Rk × K, (0, p ∞ )) where K is compact.<br />
Suppose in addition that we have k sequences f j i : [ ˜M i , ˜p i ] → [ ˜M i , ˜p i ] of diffeomorphisms<br />
of the universal covers ˜M i of M i which have the zooming in property,<br />
where ˜p i is a lift of p i , and which normalize the deck transformation group acting<br />
on ˜M i , j = 1, . . . k.<br />
Then there exists a positive integer C such that for all sufficiently large i, π 1 (M i )<br />
contains a nilpotent subgroup N ⊳ π 1 (M i ) of index at most C such that N has an<br />
(f j i )C! -invariant (j = 1, . . . , k) cyclic nilpotent chain of length ≤ n − k, that is:<br />
We can find {e} = N 0 ⊳ · · · ⊳ N n−k = N such that [N, N h ] ⊂ N h−1 and each<br />
factor group N h+1 /N h is cyclic. Furthermore, each N h is invariant under the action<br />
of (f j i )C! by conjugation and the induced automorphism of N h /N h+1 is the identity.<br />
Although we will have f j i = id in the applications, it is crucial for the proof of<br />
the theorem by induction to establish it in the stated generality.<br />
Proof. The proof proceeds by reverse induction on k. For k = n the result follows<br />
immediately from Theorem 1.2. We assume that the statement holds for all k ′ > k<br />
and we plan to prove it for k.<br />
We argue by contradiction. After passing to a subsequence we can assume that<br />
any subgroup N ⊂ π 1 (M i ) of index ≤ i does not have nilpotent cyclic chain of<br />
length ≤ n − k which is invariant under (f j i )i! (j = 1, . . . , k).<br />
After passing to a subsequence, ( ˜M i , ˜p i ) converges to (R¯k × ˜K, (0, ˜p ∞ )), where<br />
˜K contains no lines. Moreover, we can assume that the action of π 1 (M i ) converges<br />
with respect to the pointed equivariant Gromov–Hausdorff topology to an isometric<br />
action of a closed subgroup G ⊂ Iso(R¯k × ˜K).<br />
It is clear that the metric quotient (R¯k × ˜K)/G is isometric to R k ×K. Using that<br />
lines in R k × K can be lifted to lines in R¯k × ˜K, we deduce that R¯k = R k × R l and<br />
the action of G on the first Euclidean factor is trivial, (cf. proof of <strong>Lemma</strong> 2.3).<br />
Since the action of G on R l × ˜K and hence on ˜K is cocompact, we can use an
STRUCTURE OF FUNDAMENTAL GROUPS 27<br />
observation of Cheeger and Gromoll [CG72] to deduce that ˜K is compact for there<br />
are no lines in ˜K.<br />
By passing once more to a subsequence, we can assume that f j i converges in the<br />
weakly measured sense to an isometry f∞ j on the limit space, j = 1, . . . , k. The<br />
overall most difficult step is essentially a consequence of the Rescaling Theorem:<br />
Step 1. Without loss of generality we can assume that K is not a point.<br />
We assume K = pt. The strategy is to find a new contradicting sequence converging<br />
to R k × K ′ where K ′ ≠ pt.<br />
After rescaling every manifold down by a fixed factor we may assume that the<br />
limit isometry f∞ j displaces (0, p ∞ ) by less than 1/100, j = 1, . . . , k. We choose<br />
the set of ”good” points G 1 (p i ) as in Rescaling Theorem 5.1. We let G 1 (˜p i ) denote<br />
those points in B 1 (˜p i ) projecting to G 1 (p i ). By <strong>Lemma</strong> 1.6, vol(G1(˜pi))<br />
vol(B → 1.<br />
1(˜p i))<br />
By <strong>Lemma</strong> 3.5, we can remove small subsets H i ⊂ G 1 (p i ) (and the corresponding<br />
subsets from G 1 (˜p i )) such that ≤ δ i → 0 and for any choice of points<br />
vol H i<br />
vol G 1(p i)<br />
˜q i ∈ G 1 (˜p i )\H i the sequence f j i and (f j i )−1 (i ∈ N) is good on all scales at ˜q i ,<br />
j = 1, . . . , k. To simplify notations we will assume that it is already true for all<br />
choices of ˜q i ∈ G 1 (˜p i ).<br />
For all large i we can choose a point ˜q i ∈ G 1/2 (˜p i ) with f j i (˜q i) ∈ G 1 (˜p i ), j =<br />
1, . . . , k.<br />
Let q i be the image of ˜q i in M i and ¯f j i : M i → M i be the induced diffeomorphism.<br />
By Rescaling Theorem 5.1, there is a sequence g j i of isotopic to identity diffeomorphisms<br />
g j i : [λ iM i , ¯f j i (q i)] → [λ i M i , q i ]<br />
with the zooming in property. Moreover, by the same theorem, we can find lifts ˜g j i<br />
of g j i such that ˜g j i : [λ i ˜M i , f j i (˜q i)] → [λ i ˜Mi , ˜q i ]<br />
has the zooming in property. Using <strong>Lemma</strong> 3.4 we see that<br />
f j i,new := f j i ◦ ˜gj i : [λ i ˜M i , ˜q i ] → [λ i ˜Mi , ˜q i ]<br />
has the zooming in property as well for j = 1, . . . , k. Since g j i is isotopic to the<br />
identity, it follows that conjugation by ˜g j i induces an inner automorphism of π 1(M i ).<br />
Therefore f j i,new produces the same element αj i ∈ Out(π 1(M i )) as f j i .<br />
The Rescaling Theorem also ensures that π 1 (λ i M i , q i ) remains boundedly generated.<br />
Finally, it states that (λ i M i , q i ) converges to (R k × K, (0, q ∞ )) with K being<br />
a compact space with diam(K) = 10 −n2 .<br />
Thus, (λ i M i , q i ) and the maps f j i,new on the universal covers give a new contradicting<br />
sequence with the limit satisfying K ≠ pt.<br />
From now on we will assume that it’s true for the original contradicting sequence.<br />
Step 2. Without loss of generality we can assume that f j i converges in the measured<br />
sense to the identity map of the limit space R k × R l × ˜K, j = 1, . . . , k.<br />
We prove this by finite induction on j. Suppose we already found a contradicting<br />
sequence where fi 1, . . . , f j−1<br />
i converge to the identity. We have to construct one<br />
where in addition f i := f j i converges to the identity.
28 VITALI KAPOVITCH AND BURKHARD WILKING<br />
We first consider the induced diffeomorphisms ¯f i : M i → M i which converge to an<br />
isometry ¯f ∞ of R k × K. Thus, there is A ∈ O(k) and v ∈ R k such that the induced<br />
isometry of the Euclidean factor pr( ¯f ∞ ): R k → R k is given by (w ↦→ Aw + v).<br />
We claim that we can assume without loss of generality that v = 0. In fact,<br />
by Proposition 3.8, we can find a sequence of diffeomorphisms g i : M i → M i with<br />
the zooming in property such that g i converges to an isometry g ∞ which induces<br />
translation by −v on the Euclidean factor. Moreover, there is a lift ˜g i : ˜Mi → ˜M i<br />
which also has the zooming in property if we endow ˜M i with the base point ˜p i . By<br />
<strong>Lemma</strong> 3.4, we are free to replace f i by ˜g i ◦ f i and hence v = 0 without loss of<br />
generality.<br />
Since ¯f ∞ fixes the origin of the Euclidean factor, we obtain that for the limit f ∞<br />
of f i the following property holds. Given any m > 0 we can find g m ∈ G such that<br />
d(f∞(g m m (0, ˜p ∞ )), (0, ˜p ∞ )) ≤ diam(K) in R k × R l × ˜K.<br />
It is now an easy exercise to find a sequence ν b of natural numbers and a sequence<br />
of g b ∈ G for which f ν b<br />
∞ ◦ g b converges to the identity:<br />
We consider a finite ε-dense set {a 1 , . . . , a N } in the ball of radius 1 ε<br />
around<br />
(0, ˜p ∞ ) ∈ R k+l × ˜K. For each integer m choose a g m ∈ G such that<br />
d ( f m ∞(<br />
gm (0, ˜p ∞ ) ) , (0, p ∞ ) ) ≤ diam(K).<br />
The elements (f∞(g m m a 1 ), · · · , f∞(g m m a N )) are contained in B 1/ε+diam(K ′ )(0, ˜p ∞ ).<br />
Thus, there are m 1 ≠ m 2 such that d(f∞ m1 (g m1 a j ), f∞ m2 (g m2 a j )) < ε for j =<br />
1, . . . , N. Consequently, d(f∞<br />
m1−m2 (ga j ), a j ) < ε for j = 1, . . . , N with<br />
g = f∞<br />
m2−m1 gm −1<br />
1<br />
f∞ m1−m2 g m2 ∈ G.<br />
In summary, f∞ m1−m2 ◦ g displaces any point in the ball of radius 1 ε around (0, p ∞)<br />
by at most 4ε. Since ε was arbitrary, this clearly proves our claim.<br />
Let (ν b , g b ) be as above. For each b we choose a large i = i(b) ≥ 2ν b and<br />
g b ∈ π 1 (M i ) such that f ν b<br />
i ◦ g b is in the measured sense close to f ν b<br />
∞ ◦ g b . We also<br />
choose i so large that (f ν b<br />
i(b) ◦ g b) b∈N still has the zooming in property and converges<br />
to the identity. This finishes the proof of Step 2.<br />
Further Notations. We may replace p i with any other point p ∈ B 1/2 (p i ). Thus<br />
we may assume that p ∞ is a regular point in K. Let π 1 (M i , p i , r) = π 1 (M i , p i )(r)<br />
denote the subgroup of π 1 (M, p i ) generated by loops of length ≤ r. Similarly, recall<br />
that G(r) is the group generated by elements which displace (0, ˜p ∞ ) by at most r.<br />
By the Gap <strong>Lemma</strong> (2.4) there is an ε > 0 and ε i → 0 such that π 1 (M i , p i , ε) =<br />
π 1 (M i , p i , ε i ). Moreover, G(r) = G(ε) for all r ∈ (0, 2ε]. We put Γ i := π 1 (M i , p i )<br />
and Γ iε = π 1 (M i , p i , ε).<br />
Step 3. The desired contradiction arises if the index [Γ i : Γ iε ] is bounded by some<br />
constant C for all large i.<br />
If C denotes a bound on the index and d = C!, then (f j i )d leaves the subgroup Γ iε<br />
invariant and we can find a new contradicting sequence for the manifolds ˜M i /Γ iε .<br />
Thus we may assume that Γ i = Γ iε .<br />
As we have observed above, π 1 (M i , p i ) = π 1 (M i , p i , ε i ) for some ε i → 0. We now<br />
choose a sequence λ i → ∞ very slowly such that<br />
• (λ i M i , p i ) → (R k × C p∞ K, o) = (R k′ , 0) with k ′ > k.<br />
• π 1 (λ i M i , p i ) is generated by loops of length ≤ 1.
STRUCTURE OF FUNDAMENTAL GROUPS 29<br />
• f j i : λ ˜M i i → λ i ˜Mi still has the zooming in property and still converges to<br />
the identity of the limit space.<br />
We put f k+1<br />
i = · · · = fi<br />
k′ = id and obtain a new sequence contradicting our<br />
induction assumption.<br />
Step 4. There is H > 0 and a sequence of uniformly open subgroups (see Definition<br />
1.7) Υ i ⊂ Γ iε such that the index [Γ iε : Υ i ] ≤ H and such that Υ i is normalized<br />
by a subgroup of Γ i with index ≤ H for all large i.<br />
After passing to a subsequence we may assume that Γ iε converges to the limit<br />
action of a closed subgroup G ε ⊂ G. Clearly G ε contains the open subgroup G(ε) ⊂<br />
G. Since the kernel of pr is compact (as a closed subgroup of Iso( ˜K)), the images<br />
pr(G) and pr(G ε ) are closed subgroups. Moreover, pr(G ε ) is an open subgroup of<br />
pr(G) since it contains pr(G(ε)) ⊃ pr(G) 0 . Using Fukaya and Yamaguchi [FY92,<br />
Theorem 4.1] or that the component group of pr(G) is the fundamental group of the<br />
nonnegatively curved manifold pr(G)\ Iso(R l ), we see that pr(G)/ pr(G) 0 is virtually<br />
abelian.<br />
Choose a subgroup G ′ ⊳ G of finite index such that pr(G ′ )/ pr(G) 0 is free abelian.<br />
Let G ′ ε = G ′ ∩ G ε . Then pr(G) 0 ⊂ pr(G ′ ε) ⊂ pr(G ′ ). Moreover, pr(G ′ ε) is normal in<br />
pr(G ′ ) and pr(G ′ )/ pr(G ′ ε) is a finitely generated abelian group.<br />
The inverse image Ĝε := pr −1 (pr(G ε )) ∩ G ′ is a normal subgroup of G ′ with the<br />
same quotient, i.e. G ′ /Ĝε = pr(G ′ )/ pr(G ′ ε). Since the kernel of pr is compact, the<br />
open subgroup G ′ ε is cocompact in Ĝε and thus its index [Ĝε : G ′ ε] is finite.<br />
In particular, we can say that there is some integer H 1 > 0 and a subgroup G ′<br />
with [G : G ′ ] ≤ H 1 such that [G ε : (gG ε g −1 ∩ G ε )] < H 1 for all g ∈ G ′ .<br />
We want to check that there are only finitely many possibilities for gG ε g −1 ∩ G ε<br />
where g ∈ G ′ .<br />
Let g ∈ G ′ . Notice that Γ iε is a uniformly open sequence of subgroups. Choose<br />
g i ∈ Γ i with g i → g. By <strong>Lemma</strong> 1.8, Υ i := g i Γ iε g −1<br />
i ∩ Γ iε converges to gG ε g −1 ∩ G ε<br />
and the index of Υ i in Γ iε is ≤ H 1 for all large i. By Theorem 2.5, the number of<br />
generators of Γ iε is bounded by some a priori constant. Therefore, Γ iε contains at<br />
most h 0 = h 0 (n, H 1 ) subgroups of index ≤ H 1 .<br />
Combining these statements we see that<br />
{gG ε g −1 ∩ G ε | g ∈ G ′ } = {g h G ε g −1<br />
h ∩ G ε | h = 1, . . . , h 0 }<br />
for suitably chosen elements g 1 , . . . , g h0 ∈ G ′ . We choose g hi ∈ Γ i converging to<br />
g h ∈ G. By <strong>Lemma</strong> 1.8, the sequence of subgroups<br />
Υ i :=<br />
is uniformly open and converges to<br />
Υ ∞ :=<br />
⋂h 0<br />
i=1<br />
⋂h 0<br />
h=1<br />
g h G ε g −1<br />
h<br />
g hi Γ iε g −1<br />
hi<br />
=<br />
⋂<br />
g∈G ′ gG ε g −1 .<br />
Clearly, Υ ∞ ⊳G ′ . It is not hard to see that we can find elements c 1 i , . . . , cτ i ∈ Γ i that<br />
generate a subgroup of index ≤ [G : G ′ ] such that each c j i converges to an element<br />
c j ∞ ∈ G ′ . Since c j i Υ i(c j i )−1 is uniformly open and converges to c j ∞Υ ∞ (c j ∞) −1 = Υ ∞ ,<br />
one can now apply <strong>Lemma</strong> 1.8 c) to see that c j i normalizes Υ i for large i.
30 VITALI KAPOVITCH AND BURKHARD WILKING<br />
Thus, the normalizer of Υ i has finite index ≤ [G : G ′ ] in Γ i for all large i and<br />
Step 4 is established.<br />
Step 5. The desired contradiction arises if, after passing to a subsequence, the<br />
indices [Γ i : Γ iε ] ∈ N ∪ {∞} converge to ∞.<br />
By Step 4 there is a subgroup Υ i ⊂ Γ iε of index ≤ H which is normalized by a<br />
subgroup of Γ i of index ≤ H. Similarly to the beginning of the proof of Step 3, one<br />
can reduce the situation to the case of Υ i ⊳ Γ i .<br />
Moreover, Υ i ⊳ Γ i is uniformly open and hence the action of Γ i /Υ i on M i /Υ i is<br />
uniformly discrete and converges to a properly discontinuous action of the virtually<br />
abelian group G/Υ ∞ on the space R k × (R l × ˜K)/Υ ∞ . It is now easy to see that<br />
Γ iε /Υ i contains an abelian subgroup of controlled finite index for large i.<br />
After replacing Γ i once more by a subgroup of controlled finite index we may<br />
assume that Γ i /Υ i is abelian. This also shows that without loss of generality<br />
Υ i = Γ iε .<br />
Recall that by Step 2 we have assumed that f j i converges to the identity in the<br />
weakly measured sense for every j = 1, . . . , k. Therefore, the same is true for the<br />
commutator [f j i , γ i] ∈ Γ i if γ i ∈ Γ i has bounded displacement. This in turn implies<br />
that [f j i , γ i] ∈ Γ iε for all large i and j = 1, . . . , k.<br />
By Theorem 2.5, we can find for some large σ elements d i 1, . . . , d i σ ∈ Γ i with<br />
bounded displacement generating Γ i . We also assume that the first τ elements<br />
generate Γ iε for some τ < σ.<br />
Let ˆΓ i := 〈d i 1, . . . , d i σ−1〉. Note that ˆΓ i ⊳ Γ i since ˆΓ i contains Γ iε = Υ i ⊳ Γ i and<br />
Γ i /Γ iε is abelian.<br />
If for some subsequence and some integer H the group ˆΓ i has index ≤ H in Γ i<br />
then we may replace Γ i by ˆΓ i . Thus, without loss of generality, the order of the<br />
cyclic group Γ i /ˆΓ i tends to infinity. The element f k+1<br />
i := d i σ ∈ Γ i represents a<br />
generator of this factor group which has bounded displacement.<br />
We have seen above that f j i normalizes ˆΓ i and [f k+1<br />
i , f j i ] ∈ Γ iε ⊂ ˆΓ i , j = 1, . . . , k.<br />
Clearly f k+1<br />
i , being an isometry with bounded displacement, has the zooming in<br />
property.<br />
We replace M i by B i := ˜M i /ˆΓ i . Notice that (B i , ˆp i ) converges to (R k × (R l ×<br />
˜K)/Ĝ, ˆp ∞) where Ĝ is the limit group of ˆΓi . Since G/Ĝ is a noncompact group<br />
acting cocompactly on (R l × ˜K)/Ĝ, we see that (Rl × ˜K)/Ĝ splits as Rp × K ′ with<br />
K ′ compact and p > 0. If p > 1, we put f j i = id for j = k + 2, . . . , k + p.<br />
Thus the induction assumption applies: There is a positive integer C such that<br />
the following holds for all large i:<br />
We can find a subgroup N i of π 1 (B i ) of index ≤ C and a nilpotent chain of<br />
normal subgroups {e} = N 0 ⊳ . . . ⊳ N n−k−1 = N i such that the quotients are cyclic.<br />
Moreover, the groups are invariant under the automorphism induced by conjugation<br />
of (f j i )C! and the induced automorphism of N h+1 /N h is the identity, j = 1, . . . , k+1.<br />
We put d = C!, and consider the group ¯N i ⊂ π 1 (M i ) generated by (f k+1<br />
i ) d and<br />
N i . Clearly, the index of ¯N i in π 1 (M i ) is bounded by C · d.<br />
Moreover, the chain {e} = N 0 ⊳ . . . ⊳ N n−k−1 ⊳ N n−k := ¯N i is normalized by the<br />
elements (f j i )d (j = 1, . . . , k) and the action on the cyclic quotients is trivial.<br />
In other words, the sequence fulfills the conclusion of our theorem – a contradiction.<br />
□
STRUCTURE OF FUNDAMENTAL GROUPS 31<br />
Remark 6.2. If one was only interested in proving that the fundamental group<br />
contains a polycylic subgroup of controlled index, the proof would simplify considerably.<br />
First of all, one would not need any diffeomorphisms f j i to make the<br />
induction work. In the proof of Step 1 the use of the rescaling theorem could be<br />
replaced with the more elementary Product <strong>Lemma</strong> 2.1. Step 2 would become unnecessary.<br />
The core of the remaining arguments would be the same although they<br />
would simplify somewhat.<br />
7. <strong>Margulis</strong> <strong>Lemma</strong><br />
Proof of Theorem 1. Let B 1 (p) be a metric ball in complete n-manifold with Ric ><br />
−(n − 1) on B 1 (p), Ñ the universal cover of B 1 (p) and let ˜p ∈ Ñ be a lift of p.<br />
Step 1. There are universal positive constants ε 1 (n) and C 1 (n) such that the group<br />
〈 {g<br />
Γ := ∈ π1 (B 1 (p), p) ∣ d(q, gq) ≤ ε1 (n) for all q ∈ B 1/2 (˜p) }〉<br />
has a subgroup of index ≤ C 1 (n) which has a nilpotent basis of length at most n.<br />
Put N = Ñ/Γ and let ˆp denote the image of ˜p. By the definition of Γ, for each<br />
point q ∈ B 1/2 (ˆp), the fundamental group π 1 (N, q) is generated by loops of length<br />
≤ ε 1 (n). Moreover, B 3/4 (ˆp) is compact.<br />
Assume, on the contrary, that the statement is false. Then we can find a sequence<br />
of pointed n-dimensional manifolds (N i , p i ) satisfying<br />
• Ric Ni ≥ −(n − 1),<br />
• B 3/4 (p i ) is compact,<br />
• for each point q ∈ B 1/2 (p i ) the fundamental group π 1 (N i , q) is generated<br />
by loops of length ≤ 2 −i , and<br />
• π 1 (N i , p i ) does not contain a subgroup of index ≤ 2 i which has a nilpotent<br />
basis of length ≤ n.<br />
After passing to a subsequence we can assume that (N i , p i ) converges to (X, p ∞ ).<br />
We choose q i ∈ B 1/4 (p i ) such that q i converges to a regular point q ∈ X.<br />
Choose λ i → ∞ very slowly such that π 1 (λ i N i , q i ) is still generated by loops of<br />
length ≤ 1 and such that (λ i N i , q i ) converges to C q X ∼ = R k for some k ≥ 0.<br />
Notice that the rescaled sequence M i = λ i N i satisfies Ric Mi ≥ − n−1 → 0<br />
λ 2 i<br />
and B ri (q i ) is compact with r i = λi<br />
2<br />
→ ∞. Thus the existence of this sequence<br />
contradicts the Induction Theorem 6.1 with fi 1 = · · · = f i k = id.<br />
We can now finish the proof of the theorem by establishing.<br />
Step 2. Consider ε 1 (n) > 0 and Γ from Step 1. Then there are ε 2 (n), C 2 (n) > 0<br />
such that the group<br />
〈 {g<br />
H := ∈ π1 (B 1 (p), p) | d(˜p, g˜p) ≤ ε 2 (n) }〉<br />
satisfies: Γ ∩ H has index at most C 2 (n) in H.<br />
We will provide (in principle) effective bounds on ε 2 and C 2 depending on n and<br />
the (ineffective) bound ε 1 (n). Put<br />
Γ ′ :=<br />
〈 {g<br />
∈ π1 (B 1 (p), p) | d(q, gq) ≤ ε 1 (n) for all q ∈ B 3/4 (˜p) }〉 ⊂ Γ.
32 VITALI KAPOVITCH AND BURKHARD WILKING<br />
By Theorem 2.5, there exists h = h(n) such that H can be generated by some<br />
b 1 , . . . , b h ∈ H satisfying d(˜p, b i ˜p) ≤ 4ε 2 (n) for any i = 1, . . . h. We can obviously<br />
assume that the generating set {b 1 , . . . , b h } contains inverses of all its elements. We<br />
proceed in three substeps.<br />
Claim 1. There is a positive integer L(n, ε 1 (n)) such that for all ε 2 (n) < 1<br />
100L and<br />
any choice of g i ∈ {b 1 , . . . , b h }, i = 1, . . . , L we can find l, k ∈ {1, . . . , L} with l < k<br />
and g l · g l+1 · · · g k ∈ Γ ′ .<br />
We assume ε 2 (n) < 1<br />
100L<br />
and we will show that there is an a priori estimate for<br />
L. Notice that d(˜p, g 1 · · · g l ˜p) < 4ε 2 (n)l ≤ 1 25 holds for l = 1, . . . , L. Thus g 1 · · · g l<br />
maps the ball of radius B 2/3 (p) into the ball B 3/4 (p).<br />
We choose a maximal collection of points {a 1 , . . . , a m } in B 2/3 (p) with pairwise<br />
distances ≥ ε 1 (n)/4. It is immediate from Bishop–Gromov that m can be estimated<br />
just in terms of ε 1 and n.<br />
Assume we can choose g i ∈ {b 1 , . . . , b h }, i = 1, . . . , L, such that g l · · · g k ∉ Γ ′<br />
for all 1 ≤ l < k ≤ L. This implies that d(g l · · · g k a u , a u ) ≥ ε 1 (n)/4 for some<br />
u = u(l, k) ∈ {1, . . . , m}. Hence,<br />
d(g 1 · · · g k a u(l,k) , g 1 · · · g l a u(l,k) ) ≥ ε 1 (n)/4, for 1 < k < l < L.<br />
We consider the diagonal action of Γ on the m-fold product Ñ i<br />
m and the point<br />
a = (a 1 , . . . , a m ). We just showed d(g 1 · · · , g k a, g 1 · · · g l a) ≥ ε 1 (n)/4 for all 1 ≤<br />
k < l ≤ L. Thus there L points in the m-fold product B 3/4 (˜p) m which are ε 1 (n)/4<br />
separated. Now the Bishop–Gromov inequality provides an a priori bound for L.<br />
Claim 2. Choose L as in Claim 1 and assume ε 2 (n) ≤ 1<br />
w = b ν1 . . . b νl of length l ≤ L we have wΓ ′ w −1 ⊂ Γ.<br />
100L<br />
. Then for every word<br />
Let γ ′ ∈ π 1 (B 1 (p)) be an element that displaces every point in B 2/3 (˜p) by at most<br />
ε 1 (n). By the definition of Γ ′ it suffices to show that wγ ′ w −1 ∈ Γ. Let q ∈ B 1/2 (˜p).<br />
Since ε 2 (n) < 1<br />
100L , it follows that w−1 ˜p ∈ B 1/25 (˜p) and hence w −1 q ∈ B 2/3 (˜p).<br />
Therefore<br />
d(q, wγ ′ w −1 q) = d(w −1 q, γ ′ w −1 q) < ε 1 (n) for all q ∈ B 1/2 (˜p).<br />
This proves Claim 2. Step 2 now follows from<br />
Claim 3. Every element h ∈ H is of the form h = wγ with γ ∈ Γ and w = b ν1 · · · b νl<br />
with l ≤ L.<br />
We write h = w · γ, where w is a word of length l. We may assume l is chosen<br />
minimal. It suffices to prove that l ≤ L. Suppose l > L. Then we can apply<br />
Claim 1 to the tail of w and obtain w = w 1 γ ′ w 2 with γ ′ ∈ Γ ′ , w 2 a word of length<br />
< L and the word-length of w 1 · w 2 is smaller than the length of w.<br />
By Claim 2, w2 −1 γ′ w 2 ∈ Γ. Putting γ 2 = w2 −1 γ′ w 2 γ gives h = w 1 · w 2 γ 2 – a<br />
contradiction, as the length of w 1 w 2 is smaller.<br />
□<br />
Corollary 7.1. In each dimension n there exists ε > 0 such that the following<br />
holds for any complete n-manifold M and p ∈ M with Ric > −(n − 1) on B 1 (p).<br />
If the image of π 1 (B ε (p)) → π 1 (B 1 (p)) contains a nilpotent group of rank n, then<br />
M is homeomorphic to a compact infranilmanifold.
STRUCTURE OF FUNDAMENTAL GROUPS 33<br />
Actually, we will only show that M is homotopically equivalent to an infranilmanifold.<br />
By work of Farrell and Hsiang [FH83] this determines the homeomorphism<br />
type in dimensions above 4. The 4 -dimensional case follows from work of<br />
Freedman–Quinn [FQ90]. Lastly, the 3-dimensional case follows from Perelman’s<br />
solution of the geometrization conjecture.<br />
Proof of Corollary 7.1. Notice that it suffices to prove that M i is aspherical for<br />
large i: In fact, since the cohomological dimension of a rank n nilpotent group<br />
is n, the group π 1 (M i ) must then be a torsion free virtually nilpotent group of<br />
rank n and thus, by a result of Lee and Raymond [LR85], it is isomorphic to the<br />
fundamental group of an infranilmanifold. Therfore M i is homotopically equivalent<br />
to an infranilmanifold and as explained above this then gives the result.<br />
We argue by contradiction and assume that we can find ε i → 0 and a sequence<br />
of complete manifolds M i with Ric > −1 on B 1 (p i ) such that the image of<br />
π 1 (B εi (p i ), p i ) → π 1 (B 1 (p i ), p i ) contains a nilpotent group of rank n and M i is not<br />
aspherical.<br />
By the <strong>Margulis</strong> <strong>Lemma</strong> (Theorem 1), the nilpotent group can be chosen to<br />
have index ≤ C(n) in the image. Arguing on the universal cover of B 1 (p i ) it is not<br />
hard to deduce that there is δ i → 0 such that for all q i ∈ B 1/10 (p i ) the image of<br />
π 1 (B δi (q i ), q i ) → π 1 (B 1 (p i ), p i ) also contains a nilpotent subgroup of rank n.<br />
Next we observe that diam(M i , p i ) → 0. In fact, otherwise we could, after passing<br />
to a subsequence, assume that (M i , p i ) converges in the Gromov–Hausdorff sense<br />
to (Y, p ∞ ) with Y ≠ pt. Choose q i ∈ B 1/10 (p i ) converging to a regular point q ∞<br />
in the limit Y . Similarly, choose λ i ≤ 1 √ δi<br />
slowly converging to infinity such that<br />
(λ i M i , q i ) → (C q∞ Y, o) = (R k , 0) for some k > 0. Let Ñi denote the universal cover<br />
of B 1 (p i ) and ˜q i a lift of q i . Let Γ i be the subgroup of the deck transformation group<br />
generated by those elements which displace q i by at most δ i . By construction, Γ i<br />
contains a nilpotent group of rank n. Put N i = Ñi/Γ i . Since (λ i N i , q i ) → (R k , 0)<br />
with k > 0, we get a contradiction to the Induction Theorem (6.1).<br />
Thus, diam(M i ) → 0 and in particular, M i = B 1 (p i ) is a closed manifold for all<br />
large i. After a slow rescaling we may assume in addition that Ric Mi ≥ −h i → 0.<br />
Next we plan to show that the universal cover of M i converges to R n . This will<br />
follow from<br />
Claim 1. Suppose a sequence of complete n-manifolds (N i , p i ) with lower Ricci<br />
curvature bound > −h i → 0 converges to (R k × K, p ∞ ) with K compact. Also<br />
assume that π 1 (N i ) is generated by loops of length ≤ R and contains a nilpotent<br />
subgroup of rank n − k. Then the universal cover of (Ñi, ˜p i ) converges to (R n , 0).<br />
We argue by reverse induction on k. The base of induction k = n is obvious.<br />
By the Induction Theorem, after passing to a bounded cover, we may assume<br />
that π 1 (N i ) itself is a torsion free nilpotent group of rank n − k.<br />
Choose N i ⊳ π 1 (N i ) with π 1 (N i )/N i<br />
∼ = Z. Then ( Ñ i /N i , ˆp i ) converges to (R l ×<br />
K ′ , (0, p ∞ )) for some l > k and K ′ compact and the claim follows from the induction<br />
assumption.<br />
Therefore ( ˜M i , ˜p i ) converges to (R n , 0). From the Cheeger–Colding Stability Theorem<br />
(see Theorem 1.2) it follows that for any R > 0 the ball B R (p) is contractible<br />
in B R+1 (p) for all p ∈ B R (˜p i ) and i ≥ i 0 (R). Since we have a cocompact deck
34 VITALI KAPOVITCH AND BURKHARD WILKING<br />
transformation group with nearly dense orbits, the result actually holds for all<br />
p ∈ ˜M i .<br />
In order to show that M i is aspherical we may replace M i by a bounded cover and<br />
thus, by Theorem 1 without loss of generality, π 1 (M i ) has a nilpotent basis of length<br />
≤ n. Because rank(π 1 (M i )) ≥ n it follows that π 1 (M i ) is torsion free. Therefore<br />
we can choose subgroups {e} = N i 0 ⊳ · · · ⊳ N i n = π 1 (M i ) with N i j /Ni j−1 ∼ = Z.<br />
In the rest of the proof we will slightly abuse notations and sometimes drop<br />
basepoints when talking about pointed Gromov–Hausdorff convergence when the<br />
base points are clear.<br />
Note that the above claim easily implies that ˜M i /N i j → Rn−j for all j = 0, . . . n.<br />
Claim 2. N i j ⋆ B r(˜p i ) is contractible in N i j ⋆ B 4 4j r (˜p i) for j = 1, · · · , n and r ∈<br />
[<br />
1, 4<br />
4 (2n−j)2 ] and all large i.<br />
We want to prove the statement by induction on j. For j = 0 it holds as was<br />
pointed out above. Suppose it holds for j < n and we need to prove it for j + 1.<br />
Choose g ∈ N i j+1 representing a generator of Ni j+1 /Ni j .<br />
Notice that N i j+1 ⋆ B R(˜p i )/N i j converges to R × B R(0) ⊂ R n−j as i → ∞ where<br />
B R (0) is the ball in R n−j−1 . Moreover, the action of N i j+1 /Ni j on the set converges<br />
to the R action on R × B R (0) given by translations.<br />
We can also find finite index subgroups ¯N i j+1 ⊂ Ni j+1 with Ni j ⊂ ¯N i j+1 such that<br />
N i j+1 ⋆ B R(˜p i )/¯N i j+1 converges to S1 × B R (0) ⊂ S 1 × R n−j where S 1 has diameter<br />
10 −n . It is easy to construct a smooth map ¯σ : N i j+1 ⋆ B R(˜p i )/¯N i j+1 → S1 that is<br />
arbitrary close to the projection map S 1 × B R (0) → S 1 in the Gromov–Hausdorff<br />
sense.<br />
We can lift ¯σ to a map σ : N i j+1 ⋆ B R(˜p i ) → R. Notice that σ commutes with the<br />
action of ¯N i j+1 where ¯N i j+1 /Ni j can be thought of as the deck transformation group<br />
of the covering R → S 1 .<br />
It suffices to show that N i j+1 ⋆ B r(˜p i ) → N i j+1 ⋆ B 4 4j+1 r (˜p i) induces the trivial<br />
map on the level of homotopy groups. Thus, we have to show that for any k > 0<br />
any map ι: S k → N i j+1 ⋆ B r(˜p i ) is null homotopic in N i j+1 ⋆ B 4 4j+1 r (˜p i).<br />
We can assume that ι is smooth. The image of σ ◦ ι is given by an interval<br />
[a, b] in R. We choose a 10 −n -fine finite subdivision a < t 1 < · · · < t h < b of the<br />
interval such that the t α are regular values. Thus ι −1 (σ −1 (t α )) = H α is a smooth<br />
hypersurface in S k for every α.<br />
Notice that by construction, the image ι(H α ) is contained in gN j ⋆ B 2r (˜p i ) for<br />
some g ∈ ¯N i j+1 . By induction assumption, ι |H α<br />
is homotopic to a point map in<br />
gN i j ⋆ B 4 4j 2r (˜p i). We homotope ι into ˜ι such that ˜ι(H α ) is a point for all α and ˜ι is<br />
4 4j 4r close to ι.<br />
Consider now all components of H α for all α. They divide the sphere into<br />
connected regions such that each boundary component of a region is mapped by ˜ι<br />
to a point and the whole region is mapped to a set gN i j ⋆ B 4 4j 8r (˜p i) for some g.<br />
Thus the map ˜ι restricted to a region with crushed boundary components is null<br />
homotopic in g ⋆B 4 2·4 j 8r (˜p i) by the induction assumption. Clearly, this implies that<br />
ι is null homotopic in N i j+1 ⋆ B 4 4j+1 r (˜p i).<br />
This finishes the proof of Claim 2. Notice that N i n ⋆ B 1 (˜p i ) = ˜M i for large i since<br />
diam(M i ) → 0. Thus ˜M i is contractible by Claim 2.<br />
□
STRUCTURE OF FUNDAMENTAL GROUPS 35<br />
8. Almost nonnegatively curved manifolds with maximal first Betti<br />
number<br />
This is the only section where we assume lower sectional curvature bounds. The<br />
main purpose is to prove<br />
Corollary 8.1. In each dimension there are positive constants ε(n) and p 0 (n) such<br />
that for all primes p > p 0 (n) the following holds.<br />
Any manifold with diam(M, g) 2 K sec ≥ −ε(n) and b 1 (M, Z p ) ≥ n is diffeomorphic<br />
to a nilmanifold. Conversely, for every p, every compact n-dimensional nilmanifold<br />
covers another (almost flat) nilmanifold M with b 1 (M, Z p ) = n.<br />
The second part of the Corollary is fairly elementary but it provides counterexamples<br />
to a theorem of Fukaya and Yamaguchi [FY92, Corollary 0.9] which asserted<br />
that only tori should show up. The second part follows from the following <strong>Lemma</strong><br />
and the fact that every nilmanifold is almost flat, i.e. for any i > 0 it admits a<br />
metric with diam(M, g) 2 |K sec | ≤ 1 i .<br />
<strong>Lemma</strong> 8.2. Let Γ be a torsion free nilpotent group of rank n, and let p be a prime<br />
number.<br />
a) There is a subgroup of finite index Γ ′ ⊂ Γ for which we can find a surjective<br />
homomorphism Γ ′ → (Z/pZ) n .<br />
b) We can find a torsion free nilpotent group ˆΓ containing Γ as finite index<br />
subgroup such that there a surjective homomorphism ˆΓ → (Z/pZ) n .<br />
Proof. a) We argue by induction on n. The statement is trivial for n = 1. Assume<br />
it holds for n − 1. Choose a subgroup Λ ⊳ Γ with Γ/Λ ∼ = Z. By the induction<br />
assumption, we can find a subgroup Λ ′ ⊂ Λ of finite index and Λ ′′ ⊳ Λ ′ with<br />
Λ ′ /Λ ′′ ∼ = (Z/pZ) n−1 . Let g ∈ Γ represent a generator of Γ/Λ. Since Λ contains<br />
only finitely many subgroups with of index ≤ [Λ : Λ ′′ ], it follows that g l is in the<br />
normalizer of Λ ′ and Λ ′′ for a suitable l > 0. After increasing l further we may<br />
assume that the automorphism of Λ ′ /Λ ′′ ∼ = (Z/pZ) n−1 induced by the conjugation<br />
by g l is the identity.<br />
Define Γ ′ as the group generated by Λ ′ and g l . It is now easy to see that Γ ′ /Λ ′′<br />
is isomorphic to (Z/pZ) n−1 × Z and thus Γ ′ has a surjective homomorphism to<br />
(Z/pZ) n .<br />
b). An analysis of the proof of a) shows that in a) the index of Γ ′ in Γ can<br />
be bounded by a constant C only depending on n and p. Let L be the Malcev<br />
completion of Γ, i.e. L is the unique n-dimensional nilpotent Lie group containing<br />
Γ as a lattice. Let exp: l → L denote the exponential map of the group. It is<br />
well known that the group ¯Γ generated by exp( 1 C! exp−1 (Γ)) contains Γ as a finite<br />
index subgroup. By part a) we can assume now that ¯Γ contains a subgroup ˆΓ of<br />
index ≤ C such that there is a surjective homomorphism to ˆΓ → (Z/pZ) n . By<br />
construction, any subgroup of ¯Γ of index ≤ C must contain Γ and hence we are<br />
done.<br />
□<br />
<strong>Lemma</strong> 8.3. For any n there exists p(n) such that if Γ is a torsion free virtually<br />
nilpotent of rank n which admits an epimorphism onto (Z/pZ) n for some p ≥ p(n)<br />
then Γ is nilpotent.<br />
Proof. By [LR85], Γ is an almost crystallographic group, that is, there is an n-<br />
dimensional nilpotent Lie group L, a compact subgroup K ⊂ Aut(L) such that Γ
36 VITALI KAPOVITCH AND BURKHARD WILKING<br />
is isomorphic to a lattice in K ⋉ L. By a result of Auslander (see [LR85] for a<br />
proof), the projection of Γ to K is finite and we may assume that the projection<br />
is surjective. Thus K is a finite group and it is easy to see that the action of K<br />
on L/[L, L] is effective. It is known that N ′ := Γ ∩ [L, L] is a lattice in [L, L]. Thus,<br />
the image ¯Γ := Γ/N ′ of Γ in K ⋉ L/[L, L] is discrete and cocompact. Since K acts<br />
effectively on L/[L, L], we can view K ⋉ L/[L, L] as a cocompact subgroup of Iso(R k )<br />
where k = dim(L/[L, L]).<br />
In particular, ¯Γ is a crystallographic group. As the projection of Γ to K is<br />
surjective, ¯Γ is abelian if and only if K is trivial. If it is not abelian, then<br />
rank(¯Γ/[¯Γ, ¯Γ]) < rank(¯Γ) = k.<br />
By the third Bieberbach theorem there are only finitely many crystallographic<br />
groups of any given rank, and consequently there are only finitely many possibilities<br />
for the isomorphism type of ¯Γ/[¯Γ, ¯Γ]. Obviously, any homomorphism ¯Γ → (Z/pZ) k<br />
factors through ¯Γ → ¯Γ/[¯Γ, ¯Γ] → (Z/pZ) k ; this easily implies that there is p 0 such<br />
that that there is no surjective homomorphism ¯Γ → (Z/pZ) k for p ≥ p 0 .<br />
Therefore, there is no surjective homomorphism Γ → (Z/pZ) n for p > p 0 . If we<br />
put p(n) = p 0 , then ¯Γ is abelian, K = {e} and hence Γ is nilpotent.<br />
□<br />
Corollary 8.1 follows from <strong>Lemma</strong> 8.2, <strong>Lemma</strong> 8.3 and the following result<br />
Corollary 8.4. In each dimension there are positive constants C(n) and ε(n) such<br />
that for any complete manifold with K sec > −1 on B 1 (0) one of the following holds<br />
a) The image π 1 (B ε(n) (p)) → B 1 (p) contains a subgroup N of index less than<br />
C(n) such that N has a nilpotent basis of length ≤ n − 1 or<br />
b) M is compact and homeomorphic to an infranilmanifold. Moreover, any<br />
finite cover with a nilpotent fundamental group is diffeomorphic to a nilmanifold.<br />
Proof. Consider a sequence of manifolds (M i , p i ) with K sec > −1 such that M i<br />
does not satisfy a) with C = i and ε(n) = 1 i<br />
. As in proof of Corollary 7.1, it is clear<br />
that diam(M i ) → 0.<br />
By the <strong>Margulis</strong> <strong>Lemma</strong> or by work of [KPT10], we may assume that π 1 (M i ) is<br />
nilpotent. We plan to show that π 1 (M i ) has rank n for large i.<br />
The fact that π 1 (M i ) does not contain a subgroup of bounded index with a<br />
nilpotent basis of length ≤ n − 1, guarantees a certain extremal behavior if we run<br />
through the proof of the Induction Theorem. It will be clear that the blow up<br />
limits R k × K, that occur if we go through the procedure provided in the proof of<br />
the Induction Theorem, are flat – more precisely each K has to be a torus. This<br />
follows from the fact that we can never run into a situation where Step 3 of the<br />
proof of the Induction Theorem applies.<br />
In particular, if we rescale the manifolds to have diameter 1 they must converge<br />
to a torus K = T h . We can now use Yamaguchi’s fibration theorem [Yam91] to get<br />
a fibration F i → M i → T h .<br />
Let Γ i denote the kernel of π 1 (M i ) → π 1 (T h ). In the next h steps of the procedure<br />
in the proof of the Induction Theorem we replace M i by ˜M i /Γ i endowed with<br />
h deck transformations fi 1, . . . , f i h projecting to generators of π 1 (T h ).<br />
( ˆM i := ˜M i /Γ i , ˆp i ) converges to (R h , 0). The next step in the Induction Theorem<br />
would be to rescale ˆM i so that (λ i ˆMi , ˆp i ) → (R h × K ′ , ˆp ∞ ) with K ′ not a point<br />
and it will be clear again that K ′ ∼ = T h′ is a torus.
STRUCTURE OF FUNDAMENTAL GROUPS 37<br />
It is easy to see that 1 λ i<br />
is necessarily comparable to the size of the fiber F i of<br />
the Yamaguchi fibration.<br />
From the exact homotopy sequence we can deduce that Γ i = π 1 ( ˆM i ) ∼ = π 1 (F i ).<br />
The fibration theorem ensures that F i fibers over a new torus T h′ . Let Γ ′ i denote<br />
the kernel of π 1 (F i ) → π 1 (T h′ ). Clearly π 1 (M i ) has rank n if and only if Γ ′ i has<br />
rank n − h − h ′ . After finitely many similar steps this shows that π 1 (M i ) has rank<br />
n. By Corollary 7.1 the first part of b) follows. To get the second part of part b)<br />
observe that if π 1 (M i ) is nilpotent and torsion free then by the proof of [KPT10,<br />
Theorem 5.1.1], for all large i we have that M i smoothly fibers over a nilmanifold<br />
with simply connected factors. The fibers obviously have to be trivial as M i is<br />
aspherical which means that M i is diffeomorphic to a nilmanifold.<br />
□<br />
Problem. a) We suspect that in the case of almost nonnegatively curved manifolds<br />
Corollary 8.4 remains valid if one replaces the n − 1 in a) by n − 2.<br />
b) The result of this section remain valid if one just has a uniform lower bound<br />
on sectional curvature and lower Ricci curvature bounds close enough to 0.<br />
However, it would be nice to know whether or whether not they remain valid<br />
without sectional curvature assumptions.<br />
9. Finiteness results<br />
In the proof of the following theorem the work of Colding and Naber [CN10] is<br />
key.<br />
Theorem 9.1 (Normal Subgroup Theorem). Given n, D, ε 1 > 0 there are positive<br />
constants ε 0 < ε 1 and C such that the following holds:<br />
If (M, g) is a compact n-manifold M with Ric > −(n − 1) and diam(M) ≤ D,<br />
then there is ε ∈ [ε 0 , ε 1 ] and a normal subgroup N ⊳ π 1 (M) such that for all p ∈ M<br />
we have:<br />
• the image of π 1 (B ε/1000 (p), p) → π 1 (M, p) contains N and<br />
• the index of N in the image of π 1 (B ε (p), p) → π 1 (M, p) is ≤ C.<br />
To avoid confusion, we should recall that a normal subgroup N ⊳ π 1 (M, p) naturally<br />
induces a normal subgroup N ⊳ π 1 (M, q) for all p, q ∈ M.<br />
If we put ε 1 = ε Marg (n), the constant in the <strong>Margulis</strong> <strong>Lemma</strong>, then by Theorem<br />
1, the group N in the above theorem contains a nilpotent subgroup of index<br />
≤ C Marg which has a nilpotent basis of length ≤ n. After replacing N by a characteristic<br />
subgroup of controlled index Theorem 6 follows.<br />
Proof of Theorem 9.1. We will argue by contradiction. It suffices to rule out the<br />
existence of a sequence (M i , g i ) with the following properties.<br />
(1) Ric Mi > −(n − 1), diam(M i ) ≤ D;<br />
(2) for all ε ∈ (2 −i , ε 1 ) and any normal subgroup N⊳π 1 (M i ) one of the following<br />
holds<br />
(i) N is not contained in the image of π 1 (B ε/1000 (p)) for some p ∈ M i or<br />
(ii) the index of N in the image of π 1 (B ε (q i ), q i ) → π 1 (M i , q i ) is ≥ 2 i for<br />
some q i ∈ M i .<br />
Since any subsequence of (M i , g i ) also satisfies this condition, we can assume that<br />
the universal covers ( ˜M i , π 1 (M i ), ˜p i ) converge to (Y, Γ ∞ , ˜p ∞ ). Put Γ i := π 1 (M i ).
38 VITALI KAPOVITCH AND BURKHARD WILKING<br />
For δ > 0 define<br />
S i (δ) := { g ∈ Γ i | d(p, gp) ≤ δ for all p ∈ B 1/δ (˜p i ) } and Γ iδ := 〈S i (δ)〉 ⊂ Γ i<br />
for i ∈ N ∪ {∞}.<br />
By Colding and Naber [CN10], Γ ∞ is a Lie group. In particular, the component<br />
1<br />
group Γ ∞ /Γ ∞0 is discrete and hence there is some positive δ 0 ≤ min{ε 1 ,<br />
2D } such<br />
that Γ ∞δ is given by the identity component of Γ ∞ for all δ ∈ (0, 2δ 0 ).<br />
It is easy to see that this property carries over to the sequence in the following<br />
sense. We fix a large constant R ≥ 10, to be determined later, and put ε 0 := δ 0 /R.<br />
Then Γ iδ = Γ iε0 for all δ ∈ [ε 0 /1000, Rε 0 ] and all i ≥ i 0 (R).<br />
We claim that Γ iε0 is normal in Γ i . In fact, Γ i can be generated by elements<br />
which displace ˜p i by at most 2D ≤ 1 ε 0<br />
. It is straightforward to check that for any<br />
g i ∈ Γ i satisfying d(g i ˜p i , ˜p i ) ≤ 1 ε 0<br />
we have g i S(ε 0 /2)g −1<br />
i ⊂ S(ε 0 ). Thus,<br />
g i Γ iε0/2g −1<br />
i ⊂ Γ iε0 = Γ iε0/2<br />
and clearly the claim follows.<br />
In summary, Γ iε0/1000 = Γ iRε0 ⊳ Γ i with ε 0 ≤ ε 1 for all i ≥ i 0 (R). By the choice<br />
of our sequence this implies that for some q i ∈ M the index of Γ iε0 in the image of<br />
π 1 (B ε0 (q i ), q i )) → π 1 (M i ) is larger than 2 i .<br />
Similarly to Step 2 in the proof of the <strong>Margulis</strong> <strong>Lemma</strong> in section 7, this gives a<br />
contradiction provided we choose R sufficiently large. In fact, the present situation<br />
is quite a bit easier as Γ iRε0 is normal in π 1 (M i ). □<br />
<strong>Lemma</strong> 9.2. a) Let ε 0 , D, H, n > 0. Then there is a finite collection of groups<br />
such that the following holds:<br />
Let (M, g) be a compact n-dimensional manifold with diam(M) < D,<br />
Ric(M) > −(n−1) and a normal subgroup N⊳π 1 (M) and ε ≥ ε 0 . Suppose<br />
that<br />
• The image I of π 1 (B ε (q)) → π 1 (M, q) satisfies that the index [I : I ∩N]<br />
is bounded by H, for all q ∈ M.<br />
• The group N is in the image of π 1 (B ε/1000 (q 0 )) → π 1 (M, q 0 ) for some<br />
q 0 ∈ M.<br />
Then π 1 (M)/N is isomorphic to a group in the a priori chosen finite collection.<br />
b) (Bounded presentation) Given a manifold as in a) one can find finitely<br />
many loops in M based at p ∈ M whose length and number is bounded by<br />
an a priori constant such that the following holds: the loops project to a<br />
generator system of π 1 (M, p)/N and there are finitely many relations (with<br />
an a priori bound on the length of the words involved) such that these words<br />
provide a finite presentation of the group π 1 (M, p)/N.<br />
Instead of compact manifolds one could also consider manifolds satisfying the<br />
condition that the image of π 1 (B r0 (p 1 )) → π 1 (B R0 (p 1 )) is via the natural homomorphism<br />
isomorphic to π 1 (M i ), where r 0 and R 0 are given constants. In that<br />
case one would only need the Ricci curvature bound in the ball B R0+1(p 1 ). In fact,<br />
the only purpose of the Ricci curvature bound is to guarantee the existence of an<br />
ε/100–dense finite set in M with an a priori bound on the order of the set.<br />
Of course, b) implies a). However, we prove a) first and then see that b) follows<br />
from the proof. In the context of equivariant Gromov–Hausdorff convergence a
STRUCTURE OF FUNDAMENTAL GROUPS 39<br />
bounded presentation can often be carried over to the limit. That is why part b)<br />
has some advantages over a).<br />
Proof. a). The idea is to define for each such manifold a 2-dimensional finite CWcomplex<br />
C whose combinatorics is bounded by some a priori constants and which<br />
has the fundamental group π 1 (M)/N.<br />
Choose a maximal collection of points p 1 , . . . , p k in M with pairwise distances<br />
≥ ε/100. Clearly, there is an a priori bound on k depending only on n, D and ε 0 .<br />
We also may assume that p 1 = q 0 .<br />
The points p 1 , . . . , p k will represent the 0-skeleton C 0 of our CW -complex.<br />
For each point p i define F i as the finite group given by im ε/5 (p i )/(N ∩ im ε/5 (p i ))<br />
where im δ (p i ) denotes the image of π 1 (B δ (p i ), p i ) → π 1 (M, p i ). For each of the at<br />
most H elements in F i we choose a loop in B ε/5 (p i ) representing the class modulo<br />
N. We attach to p i an oriented 1-cell for each loop and call the loop the model path<br />
of the cell.<br />
For any two points p i , p j with d(p i , p j ) < ε/20 we choose in the manifold a<br />
shortest path from p i to p j . In the CW -complex we attach an edge (1-cell) between<br />
these two points. We call the chosen path the model path of the edge.<br />
This finishes the definition of the 1-skeleton C 1 of our CW -complex.<br />
For each point p i ∈ C 0 we consider the part A i of the 1-skeleton C 1 containing<br />
all points p j with d(p i , p j ) < ε/2 and also all 1-cells connecting them including the<br />
cells attached initially. If A i has more than one connected component we replace<br />
A i by the connected component containing p i .<br />
We choose loops in A i based at p i representing free generators of the free group<br />
π 1 (A i , p i ). We now consider the homomorphism π 1 (A i , p i ) → π 1 (M, p i ) induced<br />
by mapping each loop to its model path in M. The induced homomorphism<br />
α i : π 1 (A i ) → π 1 (M, p i )/N has finite image of order ≤ H since the model paths<br />
are contained in B ε (p i ).<br />
The kernel of α i is a normal subgroup of finite index ≤ H in π 1 (A i , p i ). Thus,<br />
it is finitely generated and there is an a priori bound on the number of possibilities<br />
for the kernel. We choose a free finite generator system of the kernel and attach<br />
the corresponding 2-cells to the CW -complex for each generator of the kernel.<br />
This finishes the definition of the CW -complex C.<br />
Note that by construction, the total number of cells in our complex is bounded<br />
by a constant depending only on n, D, H and ε 0 and that the attaching maps of the<br />
two cells are under control as well. Hence there are only finitely many possibilities<br />
for the homotopy type of C. Thus, we can finish the proof by establishing that<br />
π 1 (M)/N is isomorphic to π 1 (C).<br />
First notice that there is a natural surjective map π 1 (C) → π 1 (M)/N: Consider<br />
the 1-skeleton C 1 of C. Its fundamental group is free and there is a homomorphism<br />
π 1 (C 1 ) → π 1 (M) → π 1 (M)/N induced by mapping a path onto its model<br />
path. The two cells that where attached to the 1-skeleton only added relations to<br />
the fundamental group contained in the kernel of this homomorphism. Thus, the<br />
homomorphism induces a homomorphism π 1 (C, p 1 ) → π 1 (M, p 1 )/N. Moreover, it<br />
is easy to see that the homomorphism is surjective.<br />
In order to show that the homomorphism is injective we need to show that<br />
homotopies in M can be lifted in some sense to C.<br />
Suppose we have a closed curve γ in the 1-skeleton C 1 which is based at p 1 and<br />
whose model curve c 0 in M is homotopic to a curve in N ⊂ π 1 (M). We parametrize
40 VITALI KAPOVITCH AND BURKHARD WILKING<br />
γ on [0, j 0 ] for some large integer j 0 . We will assume that γ(t) = p j for some j<br />
implies t ∈ [0, j 0 ] ∩ Z – we do not assume that the converse holds.<br />
Using that p 1 = q 0 we can find a homotopy H : [0, 1] × [0, j 0 ] → M, (s, t) ↦→<br />
H(s, t) = c s (t) such that each c s is a closed loop at p 1 , c 0 is the original curve and<br />
c 1 is a curve in the ball B ε/1000 (p 1 ). If we choose j 0 large enough, we can arrange<br />
for L(c s[i,i+1] ) < ε/1000 for all i = 0, . . . , j 0 − 1, s ∈ [0, 1].<br />
After an arbitrary small change of the homotopy we can assume that c s (i) intersects<br />
the cut locus of the 0-dimensional submanifold {p 1 , . . . , p k } only finitely<br />
many times for every i = 1, . . . , j 0 − 1. We can furthermore assume that at any<br />
given parameter value s there is at most one i such that c s (i) is in the cut locus of<br />
{p 1 , . . . , p k }.<br />
For each c s (i) we choose a point ˜c s (i) := p li (s) with minimal distance to c s (i).<br />
By construction ˜c s (i) is piecewise constant in s and at each parameter value s 0<br />
at most one of the chosen points is changed. Moreover, ˜c s (0) = ˜c s (1) = p 1 and<br />
˜c 1 (i) = p 1 .<br />
We now define ˜c s|[i,i+1] in three steps. In the middle third of the interval we run<br />
through c s|[i,i+1] with triple speed. In the last third of the interval we run through<br />
a shortest path from c s (i + 1) to p li+1 (s). In the first third of the interval we run<br />
through a shortest path from p li (s) to c s (i) and if there is a choice we choose the<br />
same path that was chosen in the last third of the previous interval.<br />
Each curve ˜c s is homotopic to c s . Moreover, ˜c s (i) is piecewise constant equal to<br />
a vertex. Clearly, s ↦→ ˜c s is only piecewise continuous.<br />
The ’jumps’ for ˜c occur exactly at those parameter values at which c s (i) is the<br />
cut locus of the finite set {p 1 , . . . , p k } for some i.<br />
We introduce the following notation: for two continuous curves c 1 , c 2 in M from<br />
p to q ∈ M we say that c 1 and c 2 are homotopic modulo N if the homotopy class of<br />
the loop obtained by prolonging c 1 with the inverse parametrized curve c 2 is in N.<br />
We define a corresponding curve γ s lifting ˜c s to our CW -complex as follows:<br />
γ s|[i,i+1] should run in the one skeleton of our CW complex from ˜c s (i) to ˜c s (i+1) ∈<br />
C 0 and the model path of γ s|[i,i+1] should be modulo N homotopic to ˜c s|[i,i+1] . We<br />
choose γ s[i,i+1] in such a way that on the first half of the interval it runs through<br />
the edge from ˜c s (i) to ˜c s (i + 1) and in order to get the right homotopy type for the<br />
model path, in the second half of the interval it runs through one of the 1-cells that<br />
were attached initially to the vertex ˜c s (i + 1) ∈ C 0 .<br />
Thus, the model curve of γ s|[i,i+1] is homotopic to ˜c s|[i,i+1] modulo N. In particular,<br />
the model curve of γ s and the loop c s are homotopic modulo N.<br />
The map s ↦→ γ s is piecewise constant in s. We have to check that at the<br />
parameter where γ s jumps the homotopy type of γ s does not change.<br />
However, first we want to check that the original curve γ is homotopic to γ 0 in C.<br />
By our initial assumption, there are integers 0 = k 0 < . . . < k h = j 0 such that γ(t)<br />
is in the 0-skeleton if and only if t ∈ {k 0 , . . . , k h }. Going through the definitions<br />
above it is easy to deduce that γ(k i ) = c 0 (k i ) = ˜c 0 (k i ) = γ 0 (k i ). Thus, it suffices to<br />
check that γ |[ki,k i+1] is homotopic to γ 0|[ki,k i+1]. We have just seen that the model<br />
curve of γ 0|[ki,k i+1] is homotopic to ˜c 0|[ki,k i+1] modulo N. This curve is homotopic<br />
to c 0|[ki,k i+1], which is the model curve of γ |[ki,k i+1].<br />
Moreover, the model curve of γ |[ki,k i+1] is in an ε/5-neighbourhood of γ(k i ). By<br />
the definition of ˜c 0 this implies that ˜c 0|[ki,k i+1] is in an ε/4-neighbourhood of γ(k i ).<br />
Therefore, γ |[ki,k i+1] as well as γ 0|[ki,k i+1] only meet points of the zero skeleton
STRUCTURE OF FUNDAMENTAL GROUPS 41<br />
with distance < ε/4 to γ(k i ). Thus, they are contained in A j , where j is defined<br />
by p j = γ(k i ). Since their model curves are homotopic modulo N, our rules for<br />
attaching 2-cells imply that γ 0|[ki,k i+1] is homotopic to γ [ki,k i+1] in C.<br />
It remains to check that the homotopy type of γ s does not change at a parameter<br />
s 0 at which s ↦→ γ s is not continuous. By construction there is an i such that<br />
γ s|[0,i−1] and γ s|[i+1,j0] are independent of s ∈ [s 0 − δ, s 0 + δ] for some δ > 0.<br />
The model curve of γ s0−δ|[i−1,i+1] is modulo N homotopic to ˜c s0−δ|[i−1,i+1] which<br />
in turn is homotopic to ˜c s0+δ|[i−1,i+1] and, finally, this curve is modulo N homotopic<br />
to the model curve of γ s0+δ|[i−1,i+1].<br />
Furthermore, the model curves of γ s0±δ|[i−1,i+1] are in an ε/2-neighbourhood of<br />
˜c s0 (i − 1). Since they are homotopic modulo N, this implies by definition of our<br />
complex that γ s0−δ|[i−1,i+1] is homotopic to γ s0+δ|[i−1,i+1] in C .<br />
Thus, the curve γ is homotopic to γ 1 , which is the point curve by construction.<br />
b) Since the number of CW -complexes constructed in a) is finite, we can think<br />
of C as fixed. We choose loops in C 1 representing a free generator system of the<br />
free group π 1 (C 1 , p 1 ). The model curves of these loops represent generators of<br />
π 1 (M, p 1 )/N and the lengths of the loops are bounded.<br />
For each of the attached 2-cells we consider the loop based at some vertex p i in<br />
the one skeleton given by the attaching map. We choose a path in C 1 from p 1 to<br />
p i and conjugate the loop back to π 1 (C 1 , p 1 ).<br />
We can express this loop as word in our free generators and if collect all these<br />
words for all 2-cells, we get a finite presentation of π 1 (C, p 1 ) ∼ = π 1 (M)/N. □<br />
It will be convenient for the proof of Theorem 7 to restate it in a slightly different<br />
form.<br />
Theorem 9.3. a) Given D and n there are finitely many groups F 1 , . . . , F k<br />
such that the following holds: For any compact n-manifold M with Ric ><br />
−(n − 1) and diam(M) ≤ D we can find a nilpotent normal subgroup N ⊳<br />
π 1 (M) which has a nilpotent basis of length ≤ n − 1 such that π 1 (M)/N is<br />
isomorphic to one of the groups in our collection.<br />
b) In addition to a) one can choose a finite collection of irreducible rational<br />
representations ρ j i : F i → GL(n j i , Q) (j = 1, . . . , µ i, i = 1, . . . , k) such that<br />
for a suitable choice of the isomorphism π 1 (M)/N ∼ = F i the following holds:<br />
There is a chain of subgroups Tor(N) = N 0 ⊳ · · · ⊳ N h0 = N which are all<br />
normal in π 1 (M) such that [N, N h ] ⊂ N h−1 and N h /N h−1 is free abelian.<br />
Moroever, the action of π 1 (M) on N by conjugation induces an action of F i<br />
on N h /N h−1 and the induced representation ρ: F i → GL ( (N h /N h−1 ) ⊗ Z Q )<br />
is isomorphic to ρ j i for a suitable j = j(h), h = 1, . . . , h 0.<br />
In addition one can assume in a) that rank(N) ≤ n − 2.<br />
Proof of Theorem 9.3. We consider a contradicting sequence (M i , g i ), that is, we<br />
have diam(M i , g i ) ≤ D and Ric Mi ≥ −(n − 1) but the theorem is not true for any<br />
subsequence of (M i , g i ).<br />
We may assume that diam(M i , g i ) = D and (M i , g i ) converges to some space X.<br />
Choose p i ∈ M i converging to a regular point p ∈ X. By the Gap <strong>Lemma</strong> (2.4)<br />
there is some δ > 0 and ε i → 0 with π 1 (M i , p i , ε i ) = π 1 (M i , p i , δ).<br />
Claim. There is C and i 0 such that π 1 (M i , p i , δ) contains a subgroup of index ≤ C<br />
which has a nilpotent basis of length ≤ n − dim(X) for all large i ≥ i 0 .
42 VITALI KAPOVITCH AND BURKHARD WILKING<br />
We choose λ i < 1 ε i<br />
with λ i → ∞ such that (λ i M i , p i ) converges to (R u , 0) with<br />
u = dim(X). It is easy to see that (λ i ˜Mi /π 1 (M i , p i , ε i ), ˜p i ) converges to (R u , 0) as<br />
well. The claim now follows from the Induction Theorem (6.1) with f j i = id.<br />
We now apply the Normal Subgroup Theorem (9.1) with ε 1 = δ. There is<br />
a normal subgroup N i ⊳ π 1 (M i , g i ), some positive constants ε and C such that<br />
N i is contained in π 1 (B εi (p i )) → π 1 (M i , g i ) and N i is contained in the image of<br />
π 1 (B ε (q i ), q i ) → π 1 (M i , q i ) with index ≤ C for all q i ∈ M.<br />
By the Claim above, N i has a subgroup of bounded index with a nilpotent basis<br />
of length ≤ n − dim(X) ≤ n − 1.<br />
We are free to replace N i by a characteristic subgroup of bounded index and<br />
thus, we may assume that N i itself has a nilpotent basis of length ≤ n − dim(X).<br />
By <strong>Lemma</strong> 9.2 a), the number of possibilities for π 1 (M i )/N i is finite. In particular,<br />
part a) of Theorem 9.3 holds for the sequence and thus either b) or the<br />
addendum is the problem.<br />
By <strong>Lemma</strong> 9.2 b), we can assume that π 1 (M i , p i )/N i is boundedly presented. By<br />
passing to a subsequence we can assume that we have the same presentation for all i.<br />
Let g i1 , . . . , g iβ ∈ π 1 (M i ) denote elements with bounded displacement projecting to<br />
our chosen generator system of π 1 (M i )/N i . Moreover, there are finitely many words<br />
in g i1 , . . . , g il (independent of i) such that these words give a finite presentation of<br />
the group π 1 (M i )/N i .<br />
We can assume that g ij converges to g ∞j ∈ G ⊂ Iso(Y ) where (Y, ˜p ∞ ) is the<br />
limit of ( ˜M i , ˜p i ). Let N ∞ ⊳G be the limit of N i . By construction, G/N ∞ is discrete.<br />
Since π 1 (M i )/N i is boundedly presented, it follows that there is an epimorphism<br />
π 1 (M i )/N i → G/N ∞ induced by mapping g ij to g ∞j , for all large i.<br />
b). We plan to show that a subsequence satisfies b). We may assume that G⋆ ˜p ∞<br />
is not connected and by the Gap <strong>Lemma</strong> (2.4)<br />
(23)<br />
2ρ 0 := min{d(˜p ∞ , g˜p ∞ ) | g˜p ∞ /∈ G 0 ⋆ ˜p ∞ } > 0.<br />
Let r be the maximal nonnegative integer such that the following holds. After<br />
passing to a subsequence there is a subgroup H i ⊆ N i of rank r satisfying<br />
• H i ⊳ π 1 (M i ), N i /H i is torsion free and<br />
• there is a chain Tor(N i ) = N i0 ⊳ · · · ⊳ N ih0 = H i such that each group<br />
N ih is normal in π 1 (M i ), [N i , N ih ] ⊂ N ih−1 , N ih /N ih−1 is free abelian and<br />
for each h = 1, . . . , h 0 the induced representations of F = π 1 (M i )/N i in<br />
(N ih /N ih−1 ) ⊗ Z Q and in (N jh /N jh−1 ) ⊗ Z Q are equivalent for all i, j.<br />
Here we used implicitly that we have a natural isomorphism between π 1 (M i )/N i<br />
and π 1 (M j )/N j in order to talk about equivalent representations.<br />
Notice that these statements hold for H i = Tor(N i ). We need to prove that H i =<br />
N i . Suppose on the contrary that rank(H i ) < rank(N i ). Consider ˆM i := ˜M i /H i<br />
endowed with the action of Γ i := π 1 (M i )/H i , and let ˆp i be a lift of p i to ˆM i . By<br />
construction ˆN i := N i /H i is a torsion free normal subgroup of Γ i .<br />
We claim that there is a central subgroup A i ⊂ ˆN i of positive rank which is<br />
normal in Γ i and is generated by {a ∈ A i | d(ˆp i , aˆp i ) ≤ d i } for a sequence d i → 0:<br />
Recall that ˆN i is generated by {a ∈ ˆN i | d(ˆp i , aˆp i ) ≤ ε i }. If ˆN i is not abelian this<br />
implies that [ˆN i , ˆN i ] is generated by {a ∈ [ˆN i , ˆN i ] | d(ˆp i , aˆp i ) ≤ 4nε i }. If [ˆN i , ˆN i ] is
STRUCTURE OF FUNDAMENTAL GROUPS 43<br />
not central in ˆN i one replaces it by [ˆN i , [ˆN i , ˆN i ]]. After finitely many similar steps<br />
this proves the claim.<br />
For each positive integer l put l · A i = {g l | g ∈ A i } ⊳ Γ i . We define l i = 2 ui as<br />
the maximal power of 2 such that there is an element in l i · A i which displaces ˆp i<br />
by at most ρ 0 . Thus, any element in L i := l i · A i displaces ˆp i by at least ρ 0 /2.<br />
After passing to a subsequence we may assume that ( ˆM i , Γ i , ˆp i ) converges to<br />
(Ŷ , Ĝ, ˆp ∞) and the action of L i converges to an action of some discrete abelian<br />
subgroup L ∞ ⊳ Ĝ. Finally we let ˆN ∞ ⊳ Ĝ denote the limit group of ˆN i . Let g i ∈ L i<br />
be an element which displaces ˆp i by at most ρ 0 . Combining d(gi k ˆp i, g k+1<br />
i ˆp i ) ≤ ρ 0<br />
with our choice of ρ 0 , see (23), gives that the sets {gi k ˆp i | k ∈ Z} converge to a<br />
discrete subset in the identity component of the limit orbit Ĝ0 ⋆ ˆp ∞ . Therefore,<br />
{g ∈ L ∞ | g ⋆ ˆp ∞ ∈ Ĝ0 ⋆ ˆp ∞ }<br />
is discrete and infinite. Let K ⊂ Ĝ denote the isotropy group of ˆp ∞. By Colding and<br />
Naber K is a Lie group and thus it only has finitely many connected components.<br />
Hence L ′ ∞ := L ∞ ∩Ĝ0 ⊳Ĝ is infinite as well. Since the abelian group L′ ∞ is a discrete<br />
subgroup of a connected Lie group, it is finitely generated.<br />
We choose a free abelian subgroup ˆL ⊂ L ′ ∞ of positive rank which is normalized<br />
by Ĝ such that the induced representation of Ĝ in ˆL ⊗ Z Q is irreducible. Notice that<br />
ˆL ⊂ L ∞ commutes with ˆN ∞ . Hence we can view this as a representation of Ĝ/ˆN ∞ .<br />
Recall that π 1 (M i )/N i<br />
∼ = Γi /ˆN i is boundedly represented. Let ĝ i1 , . . . , ĝ iβ ∈ Γ i<br />
denote the images of g i1 , . . . , g iβ . There is an epimorphism Γ i /ˆN i → Ĝ/ˆN ∞ induced<br />
by sending ĝ im to its limit element ĝ ∞m ∈ Ĝ for all large i. Thus, ˆL is also naturally<br />
endowed with a representation of Γ i /ˆN i .<br />
Let b 1 , . . . , b l ∈ ˆL ∼ = Z l be a basis. For large i there are unique elements h i (b j ) ∈<br />
L i which are close to b j , j = 1, . . . , k. We extend h i to a Z-linear map h i : ˆL → L i .<br />
We plan to prove that h i : ˆL → L i is equivariant for large i. For any given<br />
linear combination ∑ l<br />
α=1 z αb α (z j ∈ Z) we know that ∑ l<br />
α=1 z αh i (b α ) is the unique<br />
element in L i which is close to ∑ lα=1 z αb α for all large i.<br />
For each ĝ ∞m and each b j we have ĝ ∞m b j ĝ∞m −1 = ∑ lα=1 z αb α for z α ∈ Z (we<br />
(∑ l<br />
suppress the dependence on m and j). We have just seen that h i α=1 z )<br />
αb α is<br />
close to ĝ ∞m b j ĝ∞m −1 for all large i. On the other hand, ĝ im h i (b j )ĝ −1<br />
im ∈ L i is the<br />
unique element in L i close to ĝ ∞m b j ĝ∞m −1 for all large i.<br />
In summary, h i (ĝ ∞m b j ĝ∞m) −1 = ĝ im h i (b j )ĝ −1<br />
im for m = 1, . . . , β, j = 1, . . . , k and<br />
all large i. This shows that h i is equivariant with respect to the representation.<br />
Thus, h i (ˆL) is a normal subgroup of Γ i and the induced representation of F =<br />
π 1 (M i )/N i = Γ/ˆN i in h i (ˆL) ⊗ Z Q and is isomorphic to the one in h j (ˆL) ⊗ Z Q for all<br />
large i, j.<br />
There is a unique subgroup A ′ i of ˆN i such that h i (ˆL) has finite index in A ′ i and<br />
ˆN i /A ′ i is torsion free. Let N ih 0+1 ⊳ N i denote the inverse image of A ′ i ⊂ ˆN i = N i /H i .<br />
Clearly, the representation of π 1 (M i )/N i in (N ih0+1/H i ) ⊗ Z Q is isomorphic to<br />
the one in (N jh0+1/H j ) ⊗ Z Q for all large i, j – a contradiction to our choice of H i .<br />
Proof of the addendum. Thus, the sequence satisfies a) and b) but not the<br />
addendum. Recall that N i has a nilpotent basis of length ≤ n − dim(X). Therefore<br />
dim(X) = 1 and N i is a torsion free group of rank n−1. Since X is one-dimensional<br />
we deduce that G/N ∞ is virtually cyclic, that is, it contains a cyclic subgroup of
44 VITALI KAPOVITCH AND BURKHARD WILKING<br />
finite index. This in turn implies that π 1 (M i )/N i is virtually cyclic and by a) we<br />
can assume that it is a fixed group F.<br />
The result (contradiction) will now follow algebraically from b): Let ρ j : F →<br />
GL(n j , Q) be a finite collection of irreducible rational representations j = 1, . . . , j 0 .<br />
Consider, for all nilpotent torsion free groups N of rank n − 1, all short exact<br />
sequences<br />
N → Γ → F<br />
with the property that there is a chain {e} = N 0 ⊳ · · · ⊳ N h0 = N such that<br />
[N, N h ] ⊂ N h−1 , each N h is normal in Γ, N h /N h−1 is free abelian group of positive<br />
rank and the induced representation of F in (N h /N h−1 )⊗ Z Q is in the finite collection.<br />
Using that F is virtually cyclic, it is now easy to see that this leaves only finitely<br />
many possibilities for the isomorphism type of Γ/N h0−1. This shows that if we<br />
replace N by N h0−1 we are still left with finitely many possibilities in a). – a<br />
contradiction.<br />
□<br />
Corollary 9.4. Given n, D there is a constant C such that any finite fundamental<br />
group of an n-manifold (M, g) with Ric > −(n − 1), diam(M) ≤ D contains a<br />
nilpotent subgroup of index ≤ C which has a nilpotent basis of length ≤ (n − 1).<br />
Example 2. Our theorems rule out some rather innocuous families of groups as<br />
fundamental groups of manifolds with lower Ricci curvature bounds.<br />
a) Consider a homomorphism h: Z → GL(2, Z) whose image does not contain<br />
a unipotent subgroup of finite index. Put h d (x) = h(dx) and consider the<br />
group Z ⋉ hd Z 2 . By part b) of Theorem 9.3 the following holds:<br />
In a given dimension n and for a fixed diameter bound D only finitely<br />
many of these groups can be realized as fundamental groups of manifold<br />
with Ric > −(n − 1) and diam(M) ≤ D.<br />
b) Consider the action of Z p k−1 on Z p k induced by 1 ↦→ 1 + p. In a given<br />
dimension n we have that for k ≥ n and p > C Marg (constant in the<br />
<strong>Margulis</strong> <strong>Lemma</strong>) the group Z p k−1 ⋉Z p k can not be a subgroup of a compact<br />
manifold with almost nonnegative Ricci curvature, since it does not have a<br />
nilpotent basis of length ≤ n.<br />
Similarly, by Corollary 9.4, for a given D there are only finitely many<br />
primes p such that Z p n−1 ⋉ Z p n is isomorphic to the fundamental group of<br />
an n-manifold with Ric > −(n − 1) and diam(M) ≤ D.<br />
Problem. Let D > 0. Can one find a finite collection of 3-manifolds such that for<br />
any 3-manifold M with Ric > −1 and diam(M) ≤ D, there is a manifold T in the<br />
finite collection and a finite normal covering T → M for which the covering group<br />
contains a cyclic subgroup of index ≤ 2?<br />
10. The Diameter Ratio Theorem<br />
The aim of this section is to prove Theorem 8.<br />
<strong>Lemma</strong> 10.1. Let X i be a sequence of compact inner metric spaces converging to<br />
a torus T. Then π 1 (X i ) is infinite for large i.<br />
The proof of the lemma is an easy exercise.
STRUCTURE OF FUNDAMENTAL GROUPS 45<br />
Proof of Theorem 8. Suppose, on the contrary, that (M i , g i ) is a sequence of compact<br />
manifolds with diam(M i ) = D, Ric Mi > −(n − 1), #π 1 (M i ) < ∞ and the<br />
diameter of the universal cover ˜M i tends to infinity.<br />
By Corollary 9.4, we know that π 1 (M i ) contains a subgroup of index ≤ C(n, D)<br />
which has a nilpotent basis of length ≤ (n − 1).<br />
Thus, we may assume that π 1 (M i ) itself has a nilpotent basis of length u < n.<br />
We also may assume that u is minimal with the property that a contradicting<br />
sequence exist. Put<br />
ˆM i := ˜M i /[π 1 (M i ), π 1 (M i )].<br />
Clearly [π 1 (M i ), π 1 (M i )] has a nilpotent basis of length ≤ u − 1. By construction<br />
ˆM i can not be a contradicting sequence and therefore diam( ˆM i ) → ∞.<br />
Let A i := π 1 (M i )/[π 1 (M i ), π 1 (M i )] denote the deck transformation group of the<br />
normal covering ˆM i → M i .<br />
1<br />
Claim. The rescaled sequence ˆM<br />
diam( ˆM i is precompact in the Gromov–Hausdorff<br />
i)<br />
topology and any limit space has finite Hausdorff dimension.<br />
The problem is, of course, that no lower curvature bound is available after rescaling.<br />
The claim will follow from a similar precompactness result for certain Cayley graphs<br />
of A i .<br />
Recall that diam(M i ) = D. Choose a base point ˆp i ∈ ˆM i . Let f 1 , . . . , f ki ∈ A i<br />
be an enumeration of all the elements a ∈ A i with d(ˆp i , aˆp i ) < 10D.<br />
There is no bound for k i , but clearly f 1 , . . . , f ki is a generator system of A i . We<br />
define a weighted metric on the abelian group A i as follows:<br />
⎧<br />
⎨∑k i<br />
d(e, a) := min |ν j | · d(ˆp, f j ˆp)<br />
⎩<br />
j=1<br />
∣<br />
∏k i<br />
j=1<br />
f νj<br />
j<br />
⎫<br />
⎬<br />
= a<br />
⎭<br />
for all a ∈ A i<br />
and d(a, b) = d(ab −1 , e) for all a, b ∈ A i . Note that this metric coincides with the<br />
restriction to A i of the inner metric on its Cayley graph where each edge corresponding<br />
to f j is given the length d(ˆp, f j ˆp). It is easy to see that the map<br />
ι i : A i → ˆM i , a i ↦→ a i ˆp i<br />
is a quasi isometry with uniform control on the constants involved. In fact, there<br />
is some L independent of i such that<br />
1<br />
L d(a, b) ≤ d(ι i(a), ι i (b)) ≤ Ld(a, b) for all a, b ∈ A i .<br />
and the image is of ι i is D-dense.<br />
Therefore, it suffices to show that<br />
1<br />
diam(A A i) i is precompact in the Gromov–<br />
Hausdorff topology and all limit spaces of convergent subsequences are finite dimensional.<br />
For the proof we need<br />
Subclaim. There is a R 0 > D (independent of i) such that the homomorphism<br />
h: A i → A i , x ↦→ x 2 satisfies that the R 0 neighborhood of h(B R (e)) contains<br />
(e), for all R and all i.<br />
B 3R<br />
2<br />
Using that B 10D (e) ⊂ A i is isometric to a subset of B 10D (ˆp i ), we can employ the<br />
Bishop–Gromov inequality in order to find a universal constant k such that B 10D (e)<br />
does not contain k points with pairwise distance ≥ D. Put R 0 := 10D · k.
46 VITALI KAPOVITCH AND BURKHARD WILKING<br />
There is nothing to prove if R ≤ 2R 0 /3. Suppose the statement holds for R ′ ≤<br />
R − D. We claim it holds for R.<br />
Let a ∈ B 3R/2 (e) \ B R0 (e). By the definition of the metric on A i , there are<br />
g 1 , . . . , g l ∈ A i with d(e, g j ) ≤ 10D, a = ∏ l<br />
j=1 g j and d(e, a) = ∑ l<br />
j=1 d(ˆp, g j ˆp). If<br />
there is any choice we assume in addition that l is minimal with these properties. By<br />
assumption l ≥ R0<br />
10D<br />
= k. By the choice of k, after a renumbering, we may assume<br />
that d(g 1 , g 2 ) ≤ D. Our assumption on l being minimal implies that d(e, g 1 g 2 ) ><br />
10D and we may assume 4D ≤ d(e, g 1 ) ≤ d(e, g 2 ). Thus,<br />
d ( e, g −2<br />
1 a) ≤ d ( e, (g 1 g 2 ) −1 a ) + D = d(e, a) − d(e, g 1 ) − d(e, g 2 ) + D<br />
≤ d(e, a) − 2d(e, g 1 ) + D ≤ d(e, a) − 7D.<br />
By assumption, this implies that g1 −2 a has distance ≤ R 0 to some b 2 ∈ A i with<br />
d(e, b) ≤ 3( 2 d(e, a)−2d(e, g1 )+D ) . Consequently, a has distance ≤ R 0 to g1b 2 2 ∈ A i<br />
with d(e, g 1 b) < 2 3d(e, a). This finishes the proof of the subclaim.<br />
Since the Ricci curvature of ˆMi is bounded below and the L-bilipschitz embedding<br />
ι i maps the ball B 50R0 (e) ⊂ A i to a subset of B 50R0 (ˆp i ), we can use Bishop–<br />
Gromov once more to see that there is a number Q > 0 (independent of i) such<br />
that the ball B 50R0 (e) ⊂ A i can be covered by Q balls of radius R 0 for all i.<br />
We now claim that the ball B 2R (e) ⊂ A i can be covered by Q balls of radius R for<br />
1<br />
all R ≥ 20R 0 and all i. This will clearly imply that<br />
diam(A A i) i is precompact in the<br />
Gromov–Hausdorff topology and that the limits have finite Hausdorff dimension.<br />
Consider the homomorphism<br />
h 8 : A i → A i , x ↦→ x 16 .<br />
It is obviously 16-Lipschitz since A i is abelian. Choose a maximal collection of<br />
points p 1 , . . . , p l ∈ B 2R (e) ⊂ A i with pairwise distances ≥ 2R 0 .<br />
5<br />
From the subclaim it easily follows that B 3R (e) ⊂ ⋃ l<br />
i=1 B 50R 0<br />
(h(p i )). Hence we<br />
can cover B 3R (e) by l · Q balls of radius R 0 .<br />
Consider now a maximal collection of points q 1 , . . . , q h ∈ B 2R (e) with pairwise<br />
distances ≥ R. In each of the balls B 2R (q i ) we can choose l points with pairwise<br />
5<br />
distances ≥ 2R 0 . Thus, B 3R (e) contains h · l points with pairwise distances ≥ 2R 0 .<br />
Since we have seen before that B 3R (e) can be covered by l · Q balls of radius R 0 ,<br />
this implies h ≤ Q as claimed.<br />
Thus,<br />
1<br />
diam( ˆM i)<br />
ˆM i is precompact in the Gromov–Hausdorff topology. After pass-<br />
1<br />
ing to a subsequence we may assume that ˆM<br />
diam( ˆM i → T. Notice that T comes<br />
i)<br />
with a transitive action of an abelian group. Therefore T itself has a natural group<br />
structure. Moreover, T is an inner metric space and the Hausdorff dimension of T is<br />
finite. Like Gromov in [Gro81] we can now deduce from a theorem of Montgomery<br />
Zippin [MZ55, Section 6.3] that T is a Lie group and thus, a torus.<br />
By <strong>Lemma</strong> 10.1, this shows π 1 ( ˆM i ) is infinite for large i – a contradiction.<br />
□<br />
Final Remarks<br />
We would like to mention that in [Wil11] the following partial converse of the<br />
<strong>Margulis</strong> <strong>Lemma</strong> (Theorem 1) is proved.
STRUCTURE OF FUNDAMENTAL GROUPS 47<br />
Theorem. Given C and n there exists m such that the following holds: Let ε > 0,<br />
and let Γ be a group containing a nilpotent subgroup N of index ≤ C which has a<br />
nilpotent basis of length ≤ n. Then there is a compact m-dimensional manifold M<br />
with sectional curvature K > −1 and a point p ∈ M such that Γ is isomorphic to<br />
the image of the homomorphism<br />
π 1<br />
(<br />
Bε (p), p ) → π 1 (M, p).<br />
Apart from the issue of finding the optimal dimension another difference to<br />
Theorem 1 is that this theorem uses the homomorphism to π 1 (M) rather than to<br />
π 1 (B 1 (p), p). This actually allows for more flexibility (by adding relations to the<br />
fundamental group at large distances to p). This is the reason why the following<br />
problem remains open.<br />
Problem. The most important problem in the context of the <strong>Margulis</strong> <strong>Lemma</strong><br />
for manifolds with lower Ricci curvature bound that remains open is whether or<br />
whether not one can arrange in Theorem 1 for the torsion of N to be abelian. We<br />
refer the reader to [KPT10] for some related conjectures for manifolds with almost<br />
nonnegative sectional curvature.<br />
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48 VITALI KAPOVITCH AND BURKHARD WILKING<br />
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Department of Mathematics, University of Toronto, Toronto, Ontario, Canada<br />
E-mail address: vtk@math.toronto.edu<br />
University of Münster, Einsteinstrasse 62, 48149 Münster, Germany<br />
E-mail address: wilking@math.uni-muenster.de