On the Characters and the Plancherel Formula of Nilpotent Groups ...

SIZES OF COMPACT SUBSETS OF HILBERT SPACE 323
largest possible integer 12 in A(x) satisfies n < exp (4/e2). For any x
in Oc
Let Q(c) be the number of possible sets A(x) for a given E > 0. Then
by Lemma 5.6,
N(Oc, E) < Q(c) (~/E~)~/s’ < Q(c) exp (e-“-*)
for E small enough.
(The estimate Q(e) < n41cB < exp (161~~) is clearly inadequate.)
Let s be a positive integer such that l/s < 8. For I = 0, l,..., s - 1,
let
Z,, = {j : 4.5-2r’s < log j < 4e-2(r+1)/r}.
If j E A(x) n Z,, , then
so the number of elements of A(x) n Z,, is at most e2(r-8)/8. Thus the
number of ways of choosing A(x) n Z,, is at most
[exp (46-2(r+1)/s)lr*(r-r)‘I = exp [4e-2w+1)/s E2w-8)/s] < exp (E-2u+8))s
Thus for E small enough
and
Q(E) Q 2e4 exp (s~-~(l+a)) < exp (E-S-~*),
N(Oc, c) < exp (e-2-58).
Letting 6 1 0 we get r(Oc) < 2. Q.E.D.
Next we show that EW(C) may be strictly smaller than EV(C). Let
c = oc ({2/(2~ + 1))) x qw4),
a Banach ball in H x H which of course is a separable Hilbert space.
Then subspaces adapted to C are uniquely determined, with
%, = 2p + 11, %+I = ll(n + 1).
It follows easily that EW(C) = - 7/4. Taking projections of the
ellipsoid only we get EV(C) > - 3/2. By 5.8 (a), 6.2, and 6.12 we
obtain Y(C) = 1, EV(C) = - 3/2. Thus in measuring volumes it
seems better to use EV primarily, as we have done, rather than EW,