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 ${\displaystyle X\times Y}$ and those of the spaces ${\displaystyle X}$ and ${\displaystyle Y}$. 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.

The theorem can be formulated as follows. Suppose ${\displaystyle X}$ and ${\displaystyle Y}$ are topological spaces, Then we have the three chain complexes ${\displaystyle C_{*}(X)}$, ${\displaystyle C_{*}(Y)}$, and ${\displaystyle C_{*}(X\times Y)}$. (The argument applies equally to the simplicial or singular chain complexes.) We also have the tensor product complex ${\displaystyle C_{*}(X)\otimes C_{*}(Y)}$, whose differential is, by definition,

${\displaystyle \partial _{C_{*}(X)\otimes C_{*}(Y)}(\sigma \otimes \tau )=\partial _{X}\sigma \otimes \tau +(-1)^{p}\sigma \otimes \partial _{Y}\tau }$

for ${\displaystyle \sigma \in C_{p}(X)}$ and ${\displaystyle \partial _{X}}$, ${\displaystyle \partial _{Y}}$ the differentials on ${\displaystyle C_{*}(X)}$,${\displaystyle C_{*}(Y)}$.

Then the theorem says that we have chain maps

${\displaystyle F\colon C_{*}(X\times Y)\rightarrow C_{*}(X)\otimes C_{*}(Y),\quad G\colon C_{*}(X)\otimes C_{*}(Y)\rightarrow C_{*}(X\times Y)}$

such that ${\displaystyle FG}$ is the identity and ${\displaystyle GF}$ is chain-homotopic to the identity. Moreover, the maps are natural in ${\displaystyle X}$ and ${\displaystyle Y}$. Consequently the two complexes must have the same homology:

${\displaystyle H_{*}(C_{*}(X\times Y))\cong H_{*}(C_{*}(X)\otimes C_{*}(Y)).}$

The original theorem was proven in terms of acyclic models but more mileage was gotten in a phrasing by Eilenberg and Mac Lane using explicit maps. The standard map ${\displaystyle F}$ they produce is traditionally referred to as the Alexander–Whitney map and ${\displaystyle G}$ the Eilenberg–Zilber map. The maps are natural in both ${\displaystyle X}$ and ${\displaystyle Y}$ and inverse up to homotopy: one has

${\displaystyle FG=\mathrm {id} _{C_{*}(X)\otimes C_{*}(Y)},\qquad GF-\mathrm {id} _{C_{*}(X\times Y)}=\partial _{C_{*}(X)\otimes C_{*}(Y)}H+H\partial _{C_{*}(X)\otimes C_{*}(Y)}}$

for a homotopy ${\displaystyle H}$ natural in both ${\displaystyle X}$ and ${\displaystyle Y}$ such that further, each of ${\displaystyle HH}$, ${\displaystyle FH}$, and ${\displaystyle HG}$ is zero. This is what would come to be known as a contraction or a homotopy retract datum.

The diagonal map ${\displaystyle \Delta \colon X\to X\times X}$ induces a map of cochain complexes ${\displaystyle C_{*}(X)\to C_{*}(X\times X)}$ which, followed by the Alexander–Whitney ${\displaystyle F}$ yields a coproduct ${\displaystyle C_{*}(X)\to C_{*}(X)\otimes C_{*}(X)}$ inducing the standard coproduct on ${\displaystyle H_{*}(X)}$. With respect to these coproducts on ${\displaystyle X}$ and ${\displaystyle Y}$, the map

${\displaystyle H_{*}(X)\otimes H_{*}(Y)\to H_{*}{\big (}C_{*}(X)\otimes C_{*}(Y){\big )}\ {\overset {\sim }{\to }}\ H_{*}(X\times Y)}$,

also called the Eilenberg–Zilber map, becomes a map of differential graded coalgebras. The composite ${\displaystyle C_{*}(X)\to C_{*}(X)\otimes C_{*}(X)}$ itself is not a map of coalgebras.

The Alexander–Whitney and Eilenberg–Zilber maps dualize (over any choice of commutative coefficient ring ${\displaystyle k}$ with unity) to a pair of maps

${\displaystyle G^{*}\colon C^{*}(X\times Y)\rightarrow {\big (}C_{*}(X)\otimes C_{*}(Y){\big )}^{*},\quad F^{*}\colon {\big (}C_{*}(X)\otimes C_{*}(Y){\big )}^{*}\rightarrow C^{*}(X\times Y)}$

which are also homotopy equivalences, as witnessed by the duals of the preceding equations, using the dual homotopy ${\displaystyle H^{*}}$. The coproduct does not dualize straightforwardly, because dualization does not distribute over tensor products of infinitely-generated modules, but there is a natural injection of differential graded algebras ${\displaystyle i\colon C^{*}(X)\otimes C^{*}(Y)\to {\big (}C_{*}(X)\otimes C_{*}(Y){\big )}^{*}}$ given by ${\displaystyle \alpha \otimes \beta \mapsto (\sigma \otimes \tau \mapsto \alpha (\sigma )\beta (\tau ))}$, the product being taken in the coefficient ring ${\displaystyle k}$. This ${\displaystyle i}$ induces an isomorphism in cohomology, so one does have the zig-zag of differential graded algebra maps

${\displaystyle C^{*}(X)\otimes C^{*}(X)\ {\overset {i}{\to }}\ {\big (}C_{*}(X)\otimes C_{*}(X){\big )}^{*}\ {\overset {G^{*}}{\leftarrow }}\ C^{*}(X\times X){\overset {C^{*}(\Delta )}{\to }}C^{*}(X)}$

inducing a product ${\displaystyle \smile \colon H^{*}(X)\otimes H^{*}(X)\to H^{*}(X)}$ in cohomology, known as the cup product, because ${\displaystyle H^{*}(i)}$ and ${\displaystyle H^{*}(G)}$ are isomorphisms. Replacing ${\displaystyle G^{*}}$ with ${\displaystyle F^{*}}$ so the maps all go the same way, one gets the standard cup product on cochains, given explicitly by

${\displaystyle \alpha \otimes \beta \mapsto {\Big (}\sigma \mapsto (\alpha \otimes \beta )(F^{*}\Delta ^{*}\sigma )=\sum _{p=0}^{\dim \sigma }\alpha (\sigma |_{\Delta ^{[0,p]}})\cdot \beta (\sigma |_{\Delta ^{[p,\dim \sigma ]}}){\Big )}}$,

which, since cochain evaluation ${\displaystyle C^{p}(X)\otimes C_{q}(X)\to k}$ vanishes unless ${\displaystyle p=q}$, reduces to the more familiar expression.

Note that if this direct map ${\displaystyle C^{*}(X)\otimes C^{*}(X)\to C^{*}(X)}$ of cochain complexes were in fact a map of differential graded algebras, then the cup product would make ${\displaystyle C^{*}(X)}$ a commutative graded algebra, which it is not. This failure of the Alexander–Whitney map to be a coalgebra map is an example the unavailability of commutative cochain-level models for cohomology over fields of nonzero characteristic, and thus is in a way responsible for much of the subtlety and complication in stable homotopy theory.

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 (simplicial) classifying space of a crossed complex stated but not proved in the paper by Ronald Brown and Philip J. Higgins on classifying spaces.

The Eilenberg–Zilber theorem is a key ingredient in establishing the Künneth theorem, which expresses the homology groups ${\displaystyle H_{*}(X\times Y)}$ in terms of ${\displaystyle H_{*}(X)}$ and ${\displaystyle H_{*}(Y)}$. 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.

• Acyclic model

• Eilenberg, Samuel; Zilber, Joseph A. (1953), "On Products of Complexes", American Journal of Mathematics, vol. 75, no. 1, pp. 200–204, doi:10.2307/2372629, JSTOR 2372629, MR 0052767.
• Hatcher, Allen (2002), Algebraic Topology, Cambridge University Press, ISBN 978-0-521-79540-1.
• Tonks, Andrew (2003), "On the Eilenberg–Zilber theorem for crossed complexes", Journal of Pure and Applied Algebra, vol. 179, no. 1–2, pp. 199–230, doi:10.1016/S0022-4049(02)00160-3, MR 1958384.
• Brown, Ronald; Higgins, Philip J. (1991), "The classifying space of a crossed complex", Mathematical Proceedings of the Cambridge Philosophical Society, vol. 110, pp. 95–120, CiteSeerX 10.1.1.145.9813, doi:10.1017/S0305004100070158.