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Simplicial Structures in Topology

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III.6 Homology of the Product of Two Polyhedra 149<br />

and so, for every n ≥ 1, we obta<strong>in</strong> the short exact sequence<br />

cokerλn+1 ��<br />

��<br />

Hn(K × L;Z)<br />

��<br />

��<br />

kerλn.<br />

As for chang<strong>in</strong>g the group of coefficients <strong>in</strong> homology (Sect. II.5), we prove that<br />

cokerλn+1 ∼ = ∑ (Hi(K;Z) ⊗ Hj(L;Z))<br />

i+ j=n<br />

kerλn ∼ = ∑<br />

i+ j=n−1<br />

Tor(Hi(K;Z),Hj(L;Z))<br />

and so we have the next result, known as the Künneth Theorem:<br />

(III.6.4) Theorem. For every pair of polyhedra |K| and |L| and every n ≥ 1, the<br />

follow<strong>in</strong>g short sequence of Abelian groups<br />

is exact.<br />

Exercises<br />

∑ (Hi(K;Z) ⊗ Hj(L;Z)) ��<br />

��<br />

Hn(K × L;Z)<br />

i+ j=n<br />

��<br />

��<br />

∑<br />

i+ j=n−1<br />

Tor(Hi(K;Z),Hj(L;Z))<br />

1. Prove that, for every C,C ′ ∈ Clp, the tensor product C ⊗C ′ belongs to Clp.<br />

2. Let f ∈ C(C,D) and g ∈ C(C ′ ,D ′ ) be two morphisms of given cha<strong>in</strong> complexes;<br />

prove that<br />

f ⊗ g: C ⊗C ′ → D ⊗ D ′<br />

def<strong>in</strong>ed on the generators by<br />

( f ⊗ g)(x ⊗ y)= f (x) ⊗ g(y)<br />

is a morphism of cha<strong>in</strong> complexes.<br />

3. Let C,C ′ ∈ C be two cha<strong>in</strong> complexes. Prove that<br />

def<strong>in</strong>ed on generators by the formula<br />

is an isomorphism of cha<strong>in</strong> complexes.<br />

μ : C ⊗C ′ → C ′ ⊗C<br />

μ(x ⊗ y)=(−1) |x||y| y ⊗ x

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