Thursday, 29 June 2006

at.algebraic topology - Equivariant maps inducing isomorphism in integral cohomology

I don't know of a reference off-hand but here's one way to think about it. First, one can think of Hi(X;mathbbZ)Hi(X;mathbbZ) as [X,K(mathbbZ,n)][X,K(mathbbZ,n)], the set of homotopy classes of maps. Notice that a cellular model for K(mathbbZ,n)K(mathbbZ,n) can be taken to be SnSn union higher cells that kill off the higher homotopy groups. Second, any map f:XtoYf:XtoY can be replaced by an inclusion iota:XtoMfiota:XtoMf, where MfMf is the mapping cylinder and it has the same homotopy type as that of YY. This works in the equivariant setting also. The third fact is that if any equivariant map f:XtoYf:XtoY induces an isomorphism in cellular cohomology and ff acts freely on both XX and YY then ff induces an isomorphism on equivariant cohomology as well. The equivariant cohomology can be thought of as maps from spaces to K(mathbbZ,n)K(mathbbZ,n) up to equivariant homotopy.



Now think of f:XtoYf:XtoY as in inclusion and there is a long exact sequence in cohomology
cdotstoHast(X;mathbbZ)toHast+1(Y,X;mathbbZ)toHast+1(Y;mathbbZ)toHast+1(X;mathbbZ)toHast+2(X;mathbbZ)tocdotscdotstoHast(X;mathbbZ)toHast+1(Y,X;mathbbZ)toHast+1(Y;mathbbZ)toHast+1(X;mathbbZ)toHast+2(X;mathbbZ)tocdots
which tells you in your case that Hi(Y,X;mathbbZ)=0Hi(Y,X;mathbbZ)=0 if i>i0i>i0. The kernel of Hi0(Y;mathbbZ)toHi0(X;mathbbZ)Hi0(Y;mathbbZ)toHi0(X;mathbbZ) is just the image of Hi0(Y,X;mathbbZ)Hi0(Y,X;mathbbZ) in Hi0(Y;mathbbZ)Hi0(Y;mathbbZ). Thinking of Hi0(Y,X;mathbbZ)Hi0(Y,X;mathbbZ) as relative homotopy classes of maps from (Y,X)(Y,X) to K(mathbbZ,i0)K(mathbbZ,i0). These maps only probe the i0i0 skeleton of (Y,X)(Y,X) because Hi(Y,X;mathbbZ)=0Hi(Y,X;mathbbZ)=0 if i>i0i>i0. And since any map can be made homotopic to a cellular map we need only study homotopy classes of maps from the i0i0-skeleton of (Y,X)(Y,X) to the i0i0-skeleton of K(mathbbZ,i0)K(mathbbZ,i0) which is Si0Si0. These are precisely the different ways of factoring a given equivariant map XtoSi0XtoSi0 via XstackrelftoYtoSi0XstackrelftoYtoSi0 (all upto equivariant homotopy).

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