# nLab Eilenberg-Zilber map

### Context

#### Homological algebra

homological algebra

Introduction

diagram chasing

# Contents

## Definition

Denote by

###### Definition

For $A,B \in sAb$ two simplicial abelian group, the Eilenberg-Zilber map (or Eilenberg-MacLane map or shuffle map) is the natural transformation of chain complexes

$\nabla_{A,B} \;\colon\; C(A) \otimes C(B) \longrightarrow C(A \otimes B)$

defined on two n-simplices $a \in A_p$ and $b \in B_q$ by

$\nabla_{A,B} \;\colon\; a \otimes b \;\mapsto\; \sum_{(\mu,\nu) \in Sh(p,q)} sgn(\mu,\nu) \cdot \big(s_\nu(a)\big) \otimes \big(s_\mu(b)\big) \;\; \in C_{p+q}(A \otimes B) = A_{p+q} \otimes B_{p+q} \,,$

where:

• the sum is over all $(p,q)$-shuffles

$(\mu,\nu) = (\mu_1, \cdots, \mu_p, \nu_1, \cdots, \nu_q) \,,$
• $sgn(\mu,\nu)$ is the signature of the corresponding permutation,

• the maps $s_{\mu}$ and $s_\nu$ are defined by:

$s_{\mu} \coloneqq s_{\mu_p - 1} \circ \cdots \circ s_{\mu_2 - 1} \circ s_{\mu_1 - 1}$

and

$s_{\nu} \coloneqq s_{\nu_q - 1} \circ \cdots \circ s_{\nu_2 - 1} \circ s_{\nu_1 - 1} \,.$

###### Remark

The shift in the indices in Def. is to be coherent with the convention that the shuffle $(\mu, \nu)$ is a permutation of $\{1, \dots, p+q\}$. In many references the shift disappears by making it a permutation of $\{0, \dots, p+q-1\}$, instead.

###### Remark

The sum in Def. may be understood as being over all non-degenerate simplices in the Cartesian product $\Delta[p] \times \Delta[q]$ of simplices. See at products of simplices for more on this.

###### Proposition

This Eilenberg-Zilber map (Def. ) co/restricts to the normalized chain complex inside the Moore complex, to a chain map of the form:

$\nabla_{A,B} \;\colon\; N(A) \otimes N(B) \longrightarrow N(A \otimes B) \,.$

## Properties

### Monoidal properties

###### Proposition

The Eilenberg-Zilber map (Def. ) is a lax monoidal transformation that makes $C$ and $N$ into lax monoidal functors.

See at monoidal Dold-Kan correspondence for details.

For the next statement notice that both $sAb$ and $Ch_\bullet^+$ are in fact symmetric monoidal categories.

###### Proposition

The EZ map (Def. ) is symmetric in that for all $A,B \in sAb$ the square

$\array{ C A \otimes C B &\stackrel{\sigma}{\to}& C B \otimes C A \\ {}^{\mathllap{\nabla_{A,B}}} \big\downarrow && \big\downarrow^{\mathrlap{\nabla_{B,A}}} \\ C(A\otimes B) &\stackrel{C(\sigma)}{\to}& C(B \otimes A) }$

commutes, where $\sigma$ denotes the symmetry isomorphism in $sAb$ and $Ch_\bullet^+$.

### Eilenberg-Zilber theorem

###### Proposition

Let

and denote

Then there is a deformation retraction

where

For unnormalized chain complexes, where we have a homotopy equivalence, this is the original Eilenberg-Zilber theorem (Eilenberg & Zilber 1953, Eilenberg & MacLane 1954, Thm. 2.1). The above deformation retraction for normalized chain complexes is Eilenberg & MacLane 1954, Thm. 2.1a. Both are reviewed in May 1967, Cor. 29.10. Explicit description of the homotopy operator is given in Gonzalez-Diaz & Real 1999.

## Applications

• The Eilenberg-Zilber map induces a functor from simplicial Lie algebras to dg-Lie algebras (see here).

• The Eilenberg-Zilber map controls the formula for transgression in group cohomology, see there fore more.

• In the context of filtered spaces $X_*, Y_*$ and their associated fundamental crossed complexes $\Pi X_*, \Pi Y_*$ there is a natural Eilenberg-Zilber morphism

$\eta: \Pi X_* \otimes \Pi Y_* \to \Pi (X_* \otimes Y_*)$

which is difficult to define directly because of the complications of the tensor product of crossed complexes, but has a direct definition in terms of the associated cubical homotopy $\omega$–groupoids. This morphism is an isomorphism of free crossed complexes if $X_*, Y_*$ are the skeletal filtrations of CW-complexes. For more on all this, see the book Nonabelian Algebraic Topology p. 533.

## References

The Eilenberg-Zilber map was introduced in:

following the Eilenberg-Zilber theorem of

Review and further discussion:

The specific maps introduced by Eilenberg-Mac Lane have stronger properties which for simplicial sets $K,L$ make $C(K) \otimes C(L)$ a strong deformation retract of $C(K \times L)$. This is exploited in

• Ronnie Brown, The twisted Eilenberg-Zilber theorem. Simposio di Topologia (Messina, 1964), Edizioni Oderisi, Gubbio (1965), 33–37. pdf