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Simplicial Structures in Topology

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III.6 Homology of the Product of Two Polyhedra 145<br />

The diagram<br />

is commutative. In fact,<br />

F(X)0<br />

η0<br />

��<br />

G(X)0<br />

ε ��<br />

Z<br />

ε ′<br />

��<br />

��<br />

Z<br />

¯f ε(F( f )0(x i 0 )) = ε′ G( f )0(y i 0 )=ε′ η0(F( f )0(x i 0 ))<br />

and therefore ¯f ε = ε ′ η0.<br />

We now prove that η0 is natural. For any given g ∈ C(X,Y),<br />

that is to say, the diagram<br />

η0F(g)0(F( f )0(x i 0 )) = η0F(gf)0(x i 0 )<br />

G(gf)0(y i 0)=G(g)0G( f )0(y i 0)=G(g)0η0(F( f )0(x i 0)),<br />

F(X)0<br />

η0<br />

��<br />

G(X)0 G(g)0<br />

F(g)0 ��<br />

F(Y )0<br />

¯f<br />

η0<br />

��<br />

��<br />

G(Y )0<br />

is commutative.<br />

We now construct η1. Let us consider the diagram<br />

F(M i 1)1<br />

G(M i 1 )1<br />

d ��<br />

i<br />

F(M1)0 d ′<br />

for a model M i 1 ∈ M1 and let us take<br />

η0<br />

��<br />

��<br />

i<br />

G(M1 )0<br />

η0d(x i 1) ∈ G(M i 1)1<br />

ε ��<br />

Z<br />

for every universal element x i 1 ∈ F(Mi 1 )1.<br />

Here is where the hypothesis on the acyclicity of G on the models of the set M<br />

is needed. Indeed, s<strong>in</strong>ce εd = 0, we have<br />

ε ′<br />

η0d(x i 1 ) ∈ kerε′ = imd ′<br />

¯f<br />

��<br />

��<br />

Z

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