nLab
2-crossed complex

Idea

Crossed complexes are a useful extension of crossed modules allowing not only the encoding of an algebraic model for the homotopy 2-type, but also information on the ‘complex of chains on the universal cover?’. The category of crossed complexes is a monoidal closed category equivalent to various types of strict infinity-groupoid?.

To model the homotopy 3-type of a space, we can use either a 2-crossed module or a crossed square (or various other algebraic models to be added some time in the future). A crossed complex is a ‘hybrid’, part crossed module but with a ‘tail’ which is a chain complex. What would be the ‘hybrid’ between a 2-crossed module and a chain complex? Are there examples that are easily constructed? What sort of information do they encode? Are they easy to analyse, understand, … and useful?

Definition

A 2-crossed complex is a normal complex of groups

$$\dots \to C_n\stackrel{{\partial }_n}{⟶}{C}_{n-1}⟶\dots ⟶C_0,$$\ldots \to C_n \stackrel{\partial_n}{\longrightarrow} C_{n-1} \longrightarrow \ldots \longrightarrow C_0,

together with a 2-crossed module structure given on $C_2\to C_1\to C_0$ by a Peiffer lifting function $\{-,-\}:C_1\times C_1\to C_2$, such that, on writing $\pi =\mathrm{Coker}(C_1\to C_0)$,

1. each $C_n$, $n\ge 3$ and $\mathrm{Ker}{\partial }_2$ are $\pi$-modules and the ${\partial }_n$ for $n\ge 4$, together with the codomain restriction of ${\partial }_3$, are $\pi$-module homomorphisms;

2. the $\pi$-module structure on $\mathrm{Ker}{\partial }_2$ is the action induced from the $C_0$-action on $C_2$ for which the action of ${\partial }_1{C}_1$ is trivial.

A 2-crossed complex morphism is defined in the obvious way, being compatible with all the actions, the pairings and Peiffer liftings. We will denote by $2\mathrm{Crs}$, the corresponding category.

Examples:

• As any crossed module gives a 2-crossed module with trivial Peiffer lifting (see the entry on 2-crossed module), it is not unexpected that any crossed complex can be considered as a 2-crossed complex with trivial Peiffer lifting. This gives an embedding of the category $\mathrm{Crs}$ into the category $2\mathrm{Crs}$). This embedding has a left adjoint given by ‘killing off’ the Peiffer lifting.

• Any 2-crossed module clearly gives a 2-crossed complex (with trivial ‘tail’).

From simplicial groups to 2-crossed complexes

If $G$ is a simplicial group then

$$\dots \to C(G{)}_3\to \frac{𝒩G_2}{d_0(𝒩G_3\cap D_3)}\to 𝒩G_1\to 𝒩G_0,$$\ldots \to C(G)_3 \to \frac{\mathcal{N}G_2}{d_0(\mathcal{N}G_3\cap D_3)} \to \mathcal{N}G_1\to \mathcal{N}G_0,

has the structure of a 2-crossed complex, where $𝒩G$ is the Moore complex of $G$, $D_n$ is the subgroup of $G_n$ generated by the degenerate elements, and, for $n>2$,

$$C(G{)}_n=\frac{𝒩G_n}{(𝒩G_n\cap D_n)d_0(𝒩G_{n+1}\cap D_{n+1})},$${C}(G)_{n} = \frac{\mathcal{N}G_n}{(\mathcal{N}G_n\cap D_n)d_0(\mathcal{N}G_{n+1}\cap D_{n+1})},

is the $n$-dimensional term of the crossed complex, $C(G)$, associated to the simplicial group $G$ as in the entry crossed complex (in the section From simplicial group(oid)s to crossed complexes.)

(There is an obvious extension of the group based definition above to a groupoid based one, and of this construction to one which takes as input a simplicially enriched groupoid.)

The Moore complex of a simplicial group $G$ has the structure of a 2-crossed complex if and only if for each $n>2$, $𝒩G_n\cap D_n$ is trivial. This means that the axioms of a group T-complex are almost satisfied, but not necessarily in dimension 2.