Monday, 21 January 2008

at.algebraic topology - Does homology detect chain homotopy equivalence?

Yes, this is true. Suppose C is such a chain complex of free abelian groups.



For each n, choose a splitting of the boundary map CntoBn1, so that CncongZnoplusBn1. (You can do this because Bn1, as a subgroup of a free group, is free.) For all n, you then have a sub-chain-complex cdotsto0toBntoZnto0tocdots concentrated in degrees n and n+1, and C is the direct sum of these chain complexes.



Given two such chain complexes C and D, you get a direct sum decomposition of each, and so it suffices to show that any two complexes cdotsto0toRitoFito0tocdots, concentrated in degrees n and n+1, which are resolutions of the same module M are chain homotopy equivalent; but this is some variant of the fundamental theorem of homological algebra.



This is special to abelian groups and is false for modules over a general ring.

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