Eilenberg–Zilber theorem


In mathematics, specifically in algebraic topology, the Eilenberg–Zilber theorem is an important result in establishing the link between the homology groups of a product space and those of the spaces and. The theorem first appeared in a 1953 paper in the American Journal of Mathematics by Samuel Eilenberg and Joseph A. Zilber. One possible route to a proof is the acyclic model theorem.

Statement of the theorem

The theorem can be formulated as follows. Suppose and are topological spaces, Then we have the three chain complexes,, and. We also have the tensor product complex, whose differential is, by definition,
for and, the differentials on,.
Then the theorem says that we have chain maps
such that is the identity and is chain-homotopic to the identity. Moreover, the maps are natural in and. Consequently the two complexes must have the same homology:
An important generalisation to the non-abelian case using crossed complexes is given in the paper by Andrew Tonks below. This give full details of a result on the classifying space of a crossed complex stated but not proved in the paper by Ronald Brown and Philip J. Higgins on classifying spaces.

Consequences

The Eilenberg–Zilber theorem is a key ingredient in establishing the Künneth theorem, which expresses the homology groups in terms of and. In light of the Eilenberg–Zilber theorem, the content of the Künneth theorem consists in analysing how the homology of the tensor product complex relates to the homologies of the factors.