# nLab crossed module

Content

### Context

#### Higher category theory

higher category theory

## 1-categorical presentations

#### Homological algebra

homological algebra

Introduction

diagram chasing

# Content

## Idea

The concept of crossed modules of groups (Whitehead 41, Whitehead 49) is a basic concept in homotopical algebra and homological algebra: It is (from the nPOV) a convenient way of encoding a strict 2-group $G$ in terms of a homomorphism $\partial : G_2 \to G_1$ of two ordinary groups.

From other points of view it is:

Historically, crossed modules were among the first examples of higher dimensional algebra to be studied.

## Definition

### Diagrammatic definition

###### Definition

A crossed module of groups is

• a pair of groups $G_2, G_1$,

• $G_2 \overset{ \delta }{ \longrightarrow } G_1$
• a group homomorphism from $G_1$ to the automorphism group of $G_2$:

$G_1 \overset{ \alpha }{ \longrightarrow } Aut(G_2) \,,$

which we may equivalently regard as a function

$\alpha \;\colon\; G_1 \times G_2 \longrightarrow G_2$

(out if the Cartesian product of underlying sets/objects)

that satisfies the $G_1$-action property and is such that for every $g_1 \in G_1$ it is a group automorphism of $G_2$;

such that the following diagrams commute:

$\array{ G_2 \times G_2 && \overset{ \delta \times Id }{ \longrightarrow } && G_1 \times G_2 \\ & {}_{\mathllap{Ad}} \searrow && \swarrow_{\mathrlap{\alpha}} \\ && G_2 }$

and

$\array{ G_1 \times G_2 & \stackrel{ \alpha }{ \longrightarrow } & G_2 \\ \big\downarrow^{ \mathrlap{Id \times \delta} } && \big\downarrow^{ \mathrlap{\delta} } \\ G_1 \times G_1 & \stackrel{ Ad }{ \longrightarrow } & G_1 \,, }$

where $Ad$ denotes the adjoint action of $G_2$ on itself.

###### Remark

The diagrammatic Def. makes sense internal to any cartesian monoidal category $\mathcal{C}$:

Alternatively, one can take another tack, and define crossed module objects in categories that support enough structure without using internal groups, the most general case of which, in practice, are semiabelian categories. There one considers the objects to behave ‘like groups’ in the sense that the category they form looks very much like the category of groups. Janelidze (Janelidze 2003) defined the notion of internal crossed module in a semiabelian category (so that in the prototypical example of the category of groups, they reduce to the above notion).

A key result, also due to (Janelidze 2003) and generalising the Brown-Spencer theorem from the case of ordinary crossed modules, is the following:

###### Theorem

(Janelidze’s Brown-Spencer theorem). Let $C$ be a semiabelian category. Then the category $XMod(C)$ of crossed modules in $C$ is equivalent to the category $Gpd(C)$ of internal groupoids in $C$.

Here the notion of internal groupoid is the usual diagrammatic notion.

### Definition in terms of equations

The two diagrams can be translated into equations, which may often be helpful.

• If we write the effect of acting with $g_1\in G_1$ on $g_2\in G_2$ as ${}^{g_1}g_2$, then the second diagram translates as the equation:

$\delta({}^{g_1}g_2) = g_1\delta(g_2)g_1^{-1}.$

In other words, $\delta$ is equivariant for the action of $G_1$.

• The first diagram is slightly more subtle. The group $G_2$ can act on itself in two different ways, (i) by the usual conjugation action, ${}^{g_2}g^\prime_2=g_2g^\prime_2g_2^{-1}$ and (ii) by first mapping $g_2$ down to $G_1$ and then using the action of that group back on $G_2$. The first diagram says that the two actions coincide. Equationally this gives:

${}^{\delta(g_2)}g^\prime_2 = g_2g^\prime_2g_2^{-1}.$

This equation is known as the Peiffer rule in the literature. Another way to interpret it is to rewrite it slightly:

${}^{\delta(g_2)}g^\prime_2 g_2 = g_2g^\prime_2$

The Peiffer rule can thus be seen as a ‘twisted commutativity law’ for $G_2$.

### Morphisms

For $G$ and $H$ two strict 2-groups coming from crossed modules $[G]$ and $[H]$, a morphism of strict 2-groups $f : G \to H$, and hence a morphism of crossed modules $[f] : [G] \to [H]$ is a 2-functor

$\mathbf{B}f : \mathbf{B}G \to \mathbf{B}H$

between the corresponding delooped 2-groupoids. Expressing this in terms of a diagram of the ordinary groups appearing in $[G]$ and $[H]$ yields a diagram called a butterfly. See there for more details.

## Examples

• For $H$ any group, its automorphism crossed module is

$AUT(H) := (G_2 = H, G_1 = Aut(H), \delta = Ad, \alpha = Id) \,.$

Under the equivalence of crossed modules with strict 2-groups this corresponds to the automorphism 2-group

$Aut_{Grpd}(\mathbf{B}H)$

of automorphisms in the category Grpd of groupoids on the one-object delooping groupoid $\mathbf{B}H$ of $H$.

• Almost the canonical example of a crossed module is given by a group $G$ and a normal subgroup $N$ of $G$. We take $G_2 = N$, and $G_1 = G$ with the action $\alpha$ being the conjugation action, whilst $\delta$ is the given inclusion, $N \hookrightarrow G$.

This is ‘almost canonical’, since if we replace the groups by simplicial groups $G_.$ and $N_.$, then $(\pi_0(G_.),\pi_0(N_.),\pi_0(inc))$ is a crossed module, and given any crossed module, $(C,P,\delta)$, there is a simplicial group $G_.$ and a normal subgroup $N_.$, such that the construction above gives the given crossed module up to isomorphism.

• Another standard example of a crossed module is $M \to ^0 P$ where $P$ is a group and $M$ is a $P$-module. Thus the category of modules over groups embeds in the category of crossed modules.

• If $\mu: M \to P$ is a crossed module with cokernel $G$, and $M$ is abelian, then the operation of $P$ on $M$ factors through $G$. In fact such crossed modules in which both $M$ and $P$ are abelian should not be sneezed at! A good example is $\mu: C_2 \times C_2 \to C_4$ where $C_n$ denotes the cyclic group of order $n$, $\mu$ is injective on each factor, and $C_4$ acts on the product by the twist. This crossed module has a classifying space $X$ with fundamental and second homotopy groups $C_2$ and non trivial $k$-invariant in $H^3(C_2, C_2)$, so $X$ is not a product of Eilenberg-MacLane spaces. However the crossed module is an algebraic model and so one one can do algebraic constructions with it. It gives in some ways a better feel for the space than the $k$-invariant. The higher homotopy van Kampen theorem implies that the above $X$ gives the 2-type of the mapping cone of the map of classifying spaces $BC_2 \to BC_4$.

• Suppose $F\stackrel{i}{\to}E\stackrel{p}{\to}B$ is a fibration sequence

of pointed spaces, thus $p$ is a fibration in the topological sense (lifting of paths and homotopies of paths will suffice), $F = p^{-1}(b_0)$, where $b_0$ is the basepoint of $B$. The fibre $F$ is pointed at $f_0$, say, and $f_0$ is taken as the basepoint of $E$ as well.

There is an induced map on homotopy groups

$\pi_1(F) \stackrel{\pi_1(i)}{\longrightarrow} \pi_1(E)$

and if $a$ is a loop in $E$ based at $f_0$, and $b$ a loop in $F$ based at $f_0$, then the composite path corresponding to $a b a^{-1}$ is homotopic to one wholly within $F$. To see this, note that $p(a b a^{-1})$ is null homotopic?. Pick a homotopy in $B$ between it and the constant map, then lift that homotopy back up to $E$ to one starting at $a b a^{-1}$. This homotopy is the required one and its other end gives a well defined element ${}^a b \in \pi_1(F)$ (abusing notation by confusing paths and their homotopy classes). With this action $(\pi_1(F), \pi(E), \pi_1(i))$ is a crossed module. This will not be proved here, but is not that difficult. (Of course, secretly, this example is ‘really’ the same as the previous one since a fibration of simplicial groups is just morphism that is an epimorphism in each degree, and the fibre is thus just a normal simplicial subgroup. What is fun is that this generalises to ‘higher dimensions’.)

• A particular case of this last example can be obtained from the inclusion of a subspace $A\to X$ into a pointed space $(X,x_0)$, (where we assume $x_0\in A$). We can replace this inclusion by a homotopic fibration, $\overline{A}\to X$ in ‘the standard way’, and then find that the fundamental group of its fibre is $\pi_2(X,A,x_0)$.

A deep theorem of J.H.C. Whitehead is that the crossed module

$\delta: \pi_2(A \cup \{e^2_\lambda\}_{\lambda \in \Lambda},A,x) \to \pi_1(A,x)$

is the free crossed module on the characteristic maps of the $2$-cells. One utility of this is that it enables the expression of nonabelian chains and boundaries ideas in dimensions $1$ and $2$: thus for the standard picture of a Klein Bottle formed by identifications from a square $\sigma$ the formula

$\delta \sigma = a+b-a +b$

makes sense with $\sigma$ a generator of a free crossed module; in the usual abelian chain theory we can write only $\partial \sigma =2b$, thus losing information.

Whitehead’s proof of this theorem used knot theory and transversality. The theorem is also a consequence of the $2$-dimensional Seifert-van Kampen Theorem, proved by Brown and Higgins, which states that the functor

$\Pi_2$: (pairs of pointed spaces) $\to$ (crossed modules)

preserves certain colimits (see reference below).

This last example was one of the first investigated by Whitehead and his proof appears also in a little book by Hilton; see also Nonabelian algebraic topology, however the more general result of Brown and Higgins determines also the group $\pi_2(X \cup CA,X,x)$ as a crossed $\pi_1(X,x)$ module, and then Whitehead’s result is the case with $A$ is a wedge of circles.

## References

The second axiom for a crossed module first appeared as footnote 35 on p. 422 of Whitehead’s paper:

• J.H.C. Whitehead, On adding relations to homotopy groups. Ann. of Math. (2) 42 (1941) 409–428.

Section 16 of the following paper

• J. H. C. Whitehead, Combinatorial Homotopy II, Bull. Amer. Math. Soc., 55 (1949), 453–496.

proved a key result on “Free crossed modules”. An exposition of this proof is in

• R. Brown, On the second relative homotopy group of an adjunction space: an exposition of a theorem of J.H.C. Whitehead, J. London Math. Soc._ (2) 22 (1980) 146-152 (doi:10.1112/jlms/s2-22.1.146)

• Peter J. Hilton, An Introduction to Homotopy Theory, Cambridge University Press 1953.

Note that the geometric core of the proof uses knot theory and transversality arguments which come from the “previous paper” of Whitehead:

• On adding relations to homotopy groups, Annals of Math., 42 (1941) 400-428.

The following paper

• Ronnie Brown, Philip Higgins, On the connections between the second relative homotopy groups of some related spaces, Proc. London Math. Soc. (3) 36 (1978) 193-212.

showed that the theorem of Whitehead on free crossed modules from CH II Sec 16 is a special case of a 2-dimwensional Van Kampen type Theorem for the homotopy crossed modules (over groupoids) of open unions of “connected” triples $(X,A,S$ of spaces where $S$ is a set of base points. However the proof of the main theorem uses the relation of crossed modules not to cat$^1$-groups but to “double groupoids with connections”, also proved with Spencer. Full details and references are in Part I of: