The Moore complex of a simplicial group – also known in its normalized version as the complex of normalized chains – is a chain complex whose differential is built from the face maps of the simplicial group.
Recall that a simplicial group , being in particular a Kan complex, may be thought of, in the sense of the homotopy hypothesis, as a combinatorial space equipped with a group structure. The Moore complex of is a chain complex
whose -cells are the “-disks with basepoint on their boundary” in this space, with the basepoint sitting on the identity element of the space;
the boundary map on which acts literally like a boundary map should: it sends an -disk to its boundary, read as an -disk whose entire boundary is concentrated at the identity point.
This is entirely analogous to how a crossed complex is obtained from a strict ω-groupoid. In fact it is a special case of that, as discussed at Dold-Kan correspondence in the section on the nonabelian version.
is in degree the joint kernel
of all face maps except the 0-face;
with differential given by the remaining 0-face map
an element in degree 1 is a 1-disk
an element is a 2-disk
a degree 2 element in the kernel of the boundary map is such a 2-disk that is closed to a 2-sphere
This means that is easy to take the homology of the complex, even though the groups involved may be non-abelian.
In this abelian cases are two other chain complexes naturally associated with :
The differential in def. 2 is well-defined in that it indeed squares to 0.
Using the simplicial identity for one finds:
Given a simplicial group (or in fact any simplicial set), then an element is called degenerate (or thin) if it is in the image of one of the simplicial degeneracy maps . All elements of are regarded a non-degenerate. Write
for the subgroup of which is generated by the degenerate elements (i.e. the smallest subgroup containing all the degenerate elements).
Def. 4 is indeed well defined in that the alternating face map differential respects the degenerate subcomplex.
Using the mixed simplicial identities we find that for a degenerate element, its boundary is
which is again a combination of elements in the image of the degeneracy maps.
Given a simplicial abelian group , the evident composite of natural morphisms
from the normalized chain complex, def. 1, into the alternating face map complex modulo degeneracies, def. 4, (inclusion followed by projection to the quotient) is a natural isomorphism of chain complexes.
e.g. (Goerss-Jardine, theorem III 2.1).
For a simplicial abelian group, there is a splitting
of the alternating face map complex, def. 2 as a direct sum, where the first direct summand is naturally isomorphic to the normalized chain complex of def. 1 and the second is the degenerate cells from def. 4.
Given a simplicial abelian group , then the inclusion
of the normalized chain complex, def. 1 into the full alternating face map complex, def. 2, is a natural quasi-isomorphism and in fact a natural chain homotopy equivalence, i.e. the complex is null-homotopic.
Following the proof of (Goerss-Jardine, theorem III 2.1) we look for each and each at the groups
and similarly at
the subgroup generated by the first degeneracies.
For these coincide with and with , respectively. We show by induction on that the composite
is an isomorphism of all . For this is then the desired result.
Consider the pre-composition of the map with the inclusion of the normalized chain complex, def. 1.
By theorem 1 the vertical map is a quasi-isomorphism and by prop. 2 the composite diagonal map is an isomorphism, hence in particular also a quasi-isomorphism. Since quasi-isomorphisms satisfy the two-out-of-three property, it follows that also the map in question is a quasi-isomorphism.
This is the statement of the Dold-Kan correspondence. See there for details.
for the -th simplicial homotopy group of . Notice that due to the group structure of in this case also is indeed canonically a group, not just a set.
For a simplicial abelian group there are natural isomorphisms
Both as well as are naturally categories with weak equivalences given by those morphisms that induce isomorphisms on all simplicial homotopy group and on all chain homology groups, respectively. So the above statement says that the Moore complex functor respects these weak equivalences.
In fact, it induces an equivalence of categories also on the corresponding homotopy categories. And even better, it induces a Quillen equivalence with respect to the standard model category structures that refine the structures of categories of weak equivalences. All this is discussed at Dold-Kan correspondence.
This has been established in (Carrasco-Cegarra). In fact, the analysis of the Moore complex and what is necessary to rebuild the simplicial group from its Moore complex is the origin of the abstract motion of hypercrossed complex, so our stated proposition is almost a tautology!
These Moore complexes are easily understood in low dimensions:
Suppose that is a simplicial group with Moore complex , which satisfies for , then has the structure of a 2-group. The interchange law is satisfied since the corresponding equation in is always the image of an element in , and here that must be trivial. If one thinks of the 2-group as being specified by a crossed module , then in terms of the original simplicial group, , , , and the action of on translates to an action of on using conjugation by , i.e., for and ,
Suppose next that for , then the Moore complex is a 2-crossed module.
This simplicial abelian group starts out as
(where we are indicating only the face maps for notational simplicity).
The second is the abelian group generated from the three (!) 1-simplicies in , namely the non-degenerate edge and the two degenerate cells and , hence the abelian group of formal linear combinations of the form
The two face maps act on the basis 1-cells as
Now of course most of the (infinitely!) many simplices inside are degenerate. In fact the only non-degenerate simplices are the two 0-cells and and the 1-cell . Hence the alternating face maps complex modulo degeneracies, def. 4, of is simply this:
Notice that alternatively we could consider the topological 1-simplex and its singular simplicial complex in place of the smaller , then the free simplicial abelian group of that. The corresponding alternating face map chain complex is “huge” in that in each positive degree it has a free abelian group on uncountably many generators. Quotienting out the degenerate cells still leaves uncountably many generators in each positive degree (while every singular -simplex in is “thin”, only those whose parameterization is as induced by a degeneracy map are actually regarded as degenerate cells here). Hence even after normalization the singular simplicial chain complex is “huge”. Nevertheless it is quasi-isomorphic to the tiny chain complex found above.
Original sources are
John Moore, Semi-simplicial complexes and Postnikov systems , Symposium international de topologia algebraica, Mexico 1958, p. 243].
John Moore, Semi-simplicial Complexes, seminar notes , Princeton University 1956]
There is also a never published
A proof by Cartan is in
A standard textbook reference for the abelian version is
Notice that these authors write “normalized chain complex” for the complex that elsewhere in the literature would be called just “Moore complex”, whereas what Goerss–Jardine call “Moore complex” is sometime maybe just called “alternating sum complex”.
A discussion with an emphasis of the generalization to non-abelian simplicial groups is in section 1.3.3 of
The discusson of the hypercrossed complex structure on the Moore complex of a general simplicial group is in