crossed square


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A crossed module is a bit like a normal subgroup … without being a subgroup. In fact if a crossed module has a boundary map which is a monomorphism then it is isomorphic to the inclusion crossed module of a normal subgroup.

Crossed modules model all connected homotopy 2-types (which by the looping and delooping-theorem means: all 2-groups). Crossed squares model all connected homotopy 3-types (hence all 3-groups) and correspond in much the same way to pairs of normal subgroups.

Suppose GG is a group and MM and NN are normal subgroups of GG; then of course, so is MNM \cap N. Put these groups in a square, with the inclusion maps between them. Finally note that if mMm \in M and nNn \in N, then [m,n][m,n] is in the intersection MNM \cap N. This gives you a crossed square with hh-map h(m,n)=[m,n]h(m,n) = [m,n]. Removing the condition that the inclusions are inclusions (!) gives the general form.

(The definition that follows is that given by Guin-Valery and Loday in their paper (see references). Another definition can be given that is just the case n=2n = 2 of that of crossed n-cube, for which see that entry.


A crossed square

L λ M λ μ N ν P \array{& L & {\to}^\lambda & M & \\ \lambda^\prime & \downarrow &&\downarrow & \mu\\ &N & {\to}_{\nu}& P & \\ }

consists of four morphisms of groups λ:LM\lambda: L \to M, λ:LN\lambda': L \to N, μ:MP\mu: M \to P, ν:NP\nu: N \to P, such that νλ=μλ\nu \lambda'= \mu \lambda together with actions of the group PP on L,M,NL, M, N on the left, conventionally, (and hence actions of MM on LL and NN via μ\mu and of NN on LL and MM via μ\mu) and a function h:M×NLh: M \times N \to L.

This structure shall satisfy the following axioms:

  1. the maps λ,λ\lambda, \lambda ' preserve the actions of PP; further, with the given actions, the maps λ,λ,μ,ν\lambda, \lambda',\mu, \nu and κ=μλ=μλ\kappa = \mu\lambda = \mu '\lambda ' are crossed modules;
  2. λh(m,n)=m nm 1,λh(m,n)=mnn 1\lambda h(m,n)=m^n m^{-1},\lambda 'h(m,n)= {^m}n\,n^{-1}
  3. h(λl,n)=l nl 1,h(m,λl)=mll 1h(\lambda l, n)= l^n l^{-1}, h(m,\lambda 'l)= {^m}l\, l^{-1}
  4. h(mm,n)=mh(m,n)h(m,n),h(m,nn)=h(m,n) nh(m,n) h(mm',n) = {^m}h(m',n) h(m,n), h(m,nn') = h(m,n) ^n h(m,n');
  5. h( pm,pn)=ph(m,n)h(^p m, {^p n})= {^p}h(m,n);

for all lL,m,mM,n,nNl\in L, \,m, m'\in M,\, n,n'\in N and pPp\in P.

The similarity of these axioms to commutator identities is no accident (see below).

This should be thought of as a crossed module of crossed modules (in either direction!). For instance horizontally:

L M λ μ N P \array{& L & & M & \\ \lambda^\prime & \downarrow &\to &\downarrow & \mu\\ &N & & P & \\ }

The image of this morphism is a normal sub-crossed module of (M,P,μ)(M,P,\mu), so we can form a quotient

μ¯:M/λLP/νN,\overline{\mu} : M/\lambda L \to P/\nu N,

and this is a crossed module, as is the kernal crossed module of this (horizontal) morphism.


Homotopical example

The classical homotopical example Π(X;A,B)\Pi(X;A,B) is determined by a pointed triad? (X;A,B)(X; A,B) where A,BXA,B \subseteq X, and P=π 1(AB)P = \pi_1(A \cap B), M=π 2(A,AB),N=π 2(B,AB)M = \pi_2(A, A \cap B), N = \pi_2(B, A \cap B) and L=π 3(X;A,B)L=\pi_3(X; A,B). The operations of PP are the standard ones and hh is the generalised Whitehead product. (The conventions may be slightly different from the standard ones in homotopy theory.) This can be generalised to a functor Π\Pi from squares of pointed spaces to crossed squares.

Ellis uses this construction in

  • G.J. Ellis, Crossed squares and combinatorial homotopy, Math. Z., 461 (1993) 93–110,

where the fact that that the crossed square associated to a triad is defined directly in terms of certain homotopy classes is important.

The fact that there is a van Kampen type theorem for Π\Pi implies that one calculates some nonabelian groups. It also implies that one is calculating some (pointed) homotopy 3-types.

Algebraic example

The example hinted at above has PP a group, MM and NN normal subgroups, and L=MNL = M \cap N,

MN λ M λ μ N ν P \array{& M\cap N & {\to}^\lambda & M & \\ \lambda^\prime & \downarrow &&\downarrow & \mu\\ &N & {\to}_{\nu}& P & \\ }

with all maps the evident inclusions, all actions by conjugation, and h:M×NMNh : M \times N \to M\cap N given by h(m,n)=[m,n]h(m,n) = [m,n].

Simplicial group example

If we replace each group in the algebraic example by a simplicial group, we would have a simplicial crossed square, now apply the connected component functor to that and you get a crossed square, and in fact any crossed square can be constructed up to isomorphism in this way.

If we start with a simplicial group, GG, using the decalage functor we can construct a simplicial group and two normal subgroups and thus get to the previous situation. The result can be interpreted in terms of the Moore complex as follows:

NG 2d 0(NG 3) Kerd 1 Kerd 2 G 1 \array{& \frac{NG_2}{d_0(NG_3)}& {\to} & Ker d_1 & \\ & \downarrow &&\downarrow & \\ &Ker d_2 & {\to}& G_1 & \\ }

The two morphisms with codomain G 1G_1 are inclusions, the other two are induced by d 0d_0. The hh-map can be explicitly given. It can be found in

T. Porter, n-types of simplicial groups and crossed n-cubes, Topology, 32, (1993), 5 – 24,

which also contains the discussion of the generalisation to crossed n-cubes


Relation to cat 2cat^2-groups

A crossed module μ:MP\mu: M \to P determines a cat 1cat^1-structure on the semidirect product group MPM \rtimes P. Thus to say that the above crossed square is a crossed module of crossed modules suggests that we should ask for LNMPL \rtimes N \to M \rtimes P to be a crossed module, so that there is an action which allows the big group G=(LN)(MP)G = (L \rtimes N) \rtimes (M \rtimes P) to be a cat 1cat^1-group. Then GG becomes a cat 2cat^2-group. The hh-map of the crossed square derives from a commutator in GG.

This equivalence between crossed squares and cat 2cat^2-groups confirms the completeness of the axioms for crossed squares. Notice also that to prove a diagram of crossed squares is a colimit diagram, it looks as if you have to make appallingly detailed verifications of axioms. It is much easier to prove the corresponding diagram of cat 2cat^2-groups is a colimit!

This theme of using two equivalent categories, one for conjecture and proof, the other for calculation and application to traditional invariants, runs through the story of higher homotopy van Kampen theorems.


  • D. Guin-Walery and J.-L. Loday, 1981, Obstructions à l’excision en K-théorie algèbrique, in Evanston Conference on Algebraic K-theory, 1980, volume 854 of Lecture Notes in Maths., 179 – 216, Springer.

Last revised on October 3, 2014 at 08:21:07. See the history of this page for a list of all contributions to it.