nLab Eilenberg-Zilber category

Idea

Eilenberg-Zilber categories are a special sort of generalized Reedy categories for which degeneracy maps behave particularly well.

Definition

Definition

An Eilenberg-Zilber category (or EZ-category) is a small category $R$ equipped with a function $d \colon ob(R) \to \mathbb{N}$ such that

1. For $f \colon x\to y$ a morphism of $R$:

   1. If $f$ is an isomorphism, then $deg(x)=deg(y)$.

   2. If $f$ is a noninvertible monomorphism, then $deg(x)\lt deg(y)$.

   3. If $f$ is a noninvertible split epimorphism, then $deg(x) \gt deg(y)$.

2. Every morphism factors as a split epimorphism followed by a monomorphism.

3. Any pair of split epimorphisms in $R$ has an absolute pushout.

Remark

Since a morphism is a split epimorphism if and only if its image in the presheaf category $[R^{op},Set]$ is an epimorphism, condition (2) in Def. says that the (epi, mono) factorization system of $[R^{op},Set]$ restricts to $R$ via the Yoneda embedding, while condition (3) says that the representables are closed in $[R^{op},Set]$ under pushouts of pairs of epimorphisms.

Properties

Any EZ-category is a generalized Reedy category where $R^+$ and $R^-$ are the monomorphisms and the split epimorphisms, respectively. Moreover, $R^{op}$ is also a generalized Reedy category where the definitions of $R^+$ and $R^-$ are reversed. However, the generalized Reedy model structures on contravariant functors (corresponding to the generalized Reedy structure on $R^{op}$) are generally better-behaved.

Any element of a presheaf on an EZ-category $R$ is a degeneracy of a unique nondegenerate element.

If an EZ-category is also a strict Reedy category (i.e. contains no nonidentity isomorphisms), then it is an elegant Reedy category.

Examples

Example

The category of simplices $\Delta$ is an Eilenberg–Zilber category.

Example

The wreath product of $\Delta$ and an EZ-category (also known as the $\Theta$-construction) is again an EZ-category (Bergner–Rezk, Proposition 4.3). In particular, Joyal’s category $\Theta_n$ is an EZ-category for all $n\ge0$.

Example

Segal's category $\Gamma$ (used to define Gamma-spaces) is an EZ-category (Berger & Moerdijk 2011, Examples 6.8).

Example

The category of symmetric simplices? (inhabited finite sets and their maps) is an EZ-category (Berger & Moerdijk, Examples 6.8).

Example

The cyclic category $\Lambda$ and the category of trees $\Omega$ are EZ-categories (Berger & Moerdijk, Examples 6.8).

Example

More generally, the total category $\mathbf{R}G$ of a crossed group $G$ on an EZ-category $\mathbf{R}$ whose underlying Reedy category is strict is itself an EZ-category (Berger & Moerdijk, Examples 6.8).

Example

The category of cubes $Q$ (generated by faces and degeneracies, without connections, symmetries, reversals, or diagonals) is an EZ-category (Isaacson 2010, Proposition 4.4).

Example

The category of symmetric cubes with min-connections (Isaacson, Definition 3.4, Proposition 3.11) is an EZ-category (Isaacson, Proposition 4.4).

Example

The cartesian cube category is an EZ-category (Campion, Theorem 8.12(1)).

References

• Clemens Berger, Ieke Moerdijk, On an extension of the notion of Reedy category, Mathematische Zeitschrift, 269, 2011 (arXiv:0809.3341, doi:10.1007/s00209-010-0770-x)

• Samuel Isaacson, Symmetric cubical sets, Journal of Pure and Applied Algebra, 215, 2011 (arXiv:0910.4948, doi:10.1016/j.jpaa.2010.08.001)

• Julia Bergner, Charles Rezk, Reedy categories and the Θ-construction, Mathematische Zeitschrift, 274, 2013 (arXiv:1110.1066, doi:10.1007/s00209-012-1082-0)

• Denis-Charles Cisinski, Higher Categories and homotopical algebra, Cambridge University Press, 2019, doi:10.1017/9781108588737)

• Tim Campion, Cubical sites as Eilenberg-Zilber categories, 2023, arXiv:2303.06206