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## For which spaces is homology (or cohomology) determined by the Eilenberg-Steenrod axioms

This is a spinoff of http://mathoverflow.net/questions/1750 . By an ordinary homology theory, I mean a functor on topological spaces which satisfies the usual set of axioms (e.g., excision, homotopy, infinite coproducts, & dimension axiom (with integer coefficients)).

It is well-known to the sort of people who know these sorts of things that any two such homology theories are isomorphic on any space with the homotopy type of a CW-complex. (If I remember correctly, one proves that there is a natural map from singular homology to another theory defined on all spaces, and that this map is an isomorphism for spaces homotopy equivalent to a CW-complex.)

The question is: are spaces with the homotopy type of CW-complexes the largest class of spaces for which this is so? A similar question is: given a space X not homotopy equivalent to a CW-complex, are there two homology theories which have different values on X?

(I'd like to add the tag "shape-theory" to this, since that's the area where an answer is mostly likely to be found. But I can't create new tags!)

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There is indeed a map from singular homology to any other ordinary homology theory E. This is given by taking a CW complex K with a weak equivalence from K to a given space X; the E-homology of K is then the singular homology of X so the induced map on E-homology gives the desired map. – Eric Wofsey Oct 22 2009 at 2:43
Thanks, I thought it was something like that. – Charles Rezk Oct 22 2009 at 2:53

This isn't an answer, but it's not really a comment either and this seems more like a working question. I'll put it here. (Nice question.)

I'd probably start by trying to construct "phantom" homology theories that annihilate any CW-complex, and see what forms of maps you could detect. This suggests taking a localization of the category of spaces by nullifying CW complexes and seeing what kinds of terrifying things you could find in there...?

Also, is the same question for cohomology theories significantly different?

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 The question for cohomology theories is the same; whether the answer is the same, I'm not sure, though my guess is it's the same. – Charles Rezk Oct 22 2009 at 3:24

I don't know the precise answer (because it depends on the precise definition of an homology theory you may prefer), but I would ask the question in a slightly different way (giving out the CW-structures for a while). We might try to think of the problem in the same way we solve the problem of a topological Galois theory over a space X: we shall get a Galois correspondance

{covers of X}<->{representations of π₁(X)}

iff (the topos of sheaves over) X is locally simply connected. If X is not locally simply connected, we shall get such a correspondance with the toposic π₁(X), which will be a pro-groupoid, hence something different from π₁(X) computed in the usual model category of spaces. We might think of this phenomena as a non additive version of our question.

More generally, given a space X, we might ask when do the topos-theoretic invariants and the CW-invariants (i.e. the ones obtained using the classical model structure on Top) agree. The interest of this point of view is that this will be a local problem on X, and that it will provide systematically two ways to look at a (co)homology theory: in the model category of spaces, or in the homotopy theory of (∞-)topoi. As far as I understand all this, the only thing we use in the CW-structure is the property of local contractibility: if X is locally contractible, then we can consider the set O(X) of contractible open subspaces of X (ordered by inclusion). We have then that X is the homotopy colimit of its contractible open subspaces (in the usual model category of spaces, but also in the setting of ∞-topoi). Hence the homotopy type of X (as a space and as a topos) is just the nerve of O(X), which has a canonical CW-structure (but I don't know how to prove that X has a CW-structure itself), so that uniqueness of (co)homology will hold on such an X (assuming some nice descent properties). The idea is that we can weaken these local conditions.

To set the general problem, we might fix a nice left Bousfield localization of the model category of topological spaces, which I shall denote by S. By nice left Bousfield localization, we might mean that it is obtained by a nullification (i.e. by inverting a map of shape A->pt). We might think of S as the homotopy theory of (n-1)-groupoids (in the case A=Sⁿ, for 0≤n≤∞), but we might also take A such that S corresponds to CW-complexes up to Quillen's plus construction, which is more related with our problem with singular homology. The reasons why I suggest to consider only nullifications are that 1) they are nicer because they lead to proper model categories (at least in Top), which makes computations easier, 2) in all the definitions, we shall only deal with weak equivalences of shape X->pt. The idea is to study the theory of "topoi relatively to S".

Given an ∞-topos X, let us denote by Sh(X) the ∞-category of stacks on X with values in S. We thus have Sh(pt)=S. Let us say that X is S-aspherical (or, if you prefer, S-acyclic...) if the constant stack functor S->Sh(X) is fully faithful. And let us say that X is locally S-aspherical if there exists a generating family in X (if X is a topological space, this means a basis of open subspaces) made of stacks U over X such that X/U is aspherical. For a general X, the constant stack functor c:S->Sh(X) will always have a pro-adjoint, and the nice thing is that this pro-adjoint will be a genuine left adjoint to c if and only if X is locally S-aspherical. In other words, we then have a functor à la Artin-Mazur

{∞-topoi}->Pro(S)

(which has a right adjoint; see 7.1.6.15 in Lurie's book on higher topoi), and I guess it will induce an equivalence of ∞-categories of shape

{locally S-aspherical ∞-topoi}=S

(for this, consider {locally S-aspherical ∞-topoi} as localized by S-equivalences, i.e. by maps which become invertible in Pro(S)).

Thus, for a topologcal space X, we have two ways to send it to Pro(S): consider the pro-S-homotopy type associated to Sh(X), or consider X as an object of the categoy underlying the model category structure on Top underlying the definition of S. Then, topological spaces X whose associated ∞-topos are locally S-aspherical are precisely the one for which the two ways to see them in Pro(S) coincide. If S is the homotopy theory associated to Quillen's plus construction, then we get that, for any topological space X which has a basis of open subspaces U such that U->pt induces an isomorphism in (pro)homology with constant coefficients, the topos theoretic singular (co)homology and the "usual" singular (co)homology (computed using Hom's in (pro-)spectra) will coincide. But otherwise, they won't, and we might expect to get explicit examples of non-agreement in this way.

What I call the topos theoretic singular cohomology is just the sheaf cohomology with coefficients in the constant sheaf Z (which is the same as taking the global sections of the image of the K(Z,n)'s by c:S->Sh(X)). If you consider a topological space X, then the topos-theoretic singular cohomology of X will be a cohomology theory satisfying excision, which will coincide with usual singular cohomology for locally contractible spaces (and more generaly, locally S-aspherical spaces, where S denotes the homotopy theory of spaces up to Quillen's plus construction). For homology, you will have the same picture, except that it will take values in pro-abelian groups in general (except again for locally contractible spaces, etc): consider the associated pro-space, and take its usual homology. This toposic point of view thus gives you a cohomology theory which coincides with singular (co)homology for good spaces, but won't agree for a large class of them. All this story applies for any (co)homology theory which preserves homotopy colimits (which implies the Eilenberg-Steenrod axioms).

As for the existence of the pro-adjoint of c:S->Sh(X), this is because any accessible left exact functor has such a thing (as proved in Lurie's book). The pro-homotopy type associated to X is then the image of the terminal object of Sh(X) by this pro-adjoint.

I should mention as well that the theory of pro-homotopy types of topoi is treated in section 7.1.6 of Lurie's book on infinity topoi (in the case where S is the usual homotopy theory of infinity-groupoids, but the same arguments should work in the generality I suggested here). I cannot resist to mention that this kind of point of view can be seen (after Lurie) as a conceptual way to look at shape theory, but this is also (higher) Galois theory in the sense of Grothendieck (such a point of view is also mentionned in the work of Toën and Vezzosi on higher topoi): in the case where S is the homotopy theory of 1-groupoids, one gets exactly the theory of the fundamental group as developped in SGA1 (and later by Leroy, Moerdijk, etc).

[Conjecture] I propose the following conjecture as an answer to the initial question (and the constructions suggested above give at least some evidence, not to say an idea for a strategy of proof).

The largest class of spaces for which the Eilenberg-Steenrod axioms determine singular homology consists of the topological spaces X satisfying one of the following four equivalent conditions.

1) for any open subspace U of X, sheaf-theoretic homology and usual singular homology coincide;

2) the open subspaces U of X such that the sheaf theoretic singular homology of U is trivial in degree >0 form a basis of open subsets of X;

3) the open subspaces U of X such that the usual singular homology of U is trivial in degree >0 form a basis of open subsets of X;

4) the open subspaces U of X such that U_+ is contractible form a basis of open subsets of X.

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 I'm trying to understand this; I don't really know the Artin-Mazur prohomotopy theory. Some questions: Why does c: S->Sh(X) have a pro-adjoint? What is the "topos theoretic singular (co)homology"? Is the idea that {infty-topoi} -> Pro(S) captures everything needed to compute cohomology using the Eilenberg-Steenrod axioms (for your Sh(X) when X is a space)? – Charles Rezk Oct 28 2009 at 0:38 I have tried to give (a little) more references about the existence of pro-adjoints above. The idea is that, using the functor {infinity-topoi}->Pro(S), one sees that sheaf-theoretic singular (co)homology of a locally nice (e.g. contractible) space is canonically isomorphic to singular cohomology of a CW-complex (the nerve of the poset of suitable open subspaces), hence coincides with the usual thing, while, in general, it satisfies the axioms you want, but does not agree with usual (co)homology. – Denis-Charles Cisinski Oct 29 2009 at 16:41 It sounds like you are saying something like this: that singular homology and "sheaf-theoretic singular homology" are the two extreme examples of Eilenberg-Steenrod type theories, so that any other Eilenberg-Steenrod theory is sandwiched between them. If so, then statement (1) of your conjecture would seem to follow. Is this what you are suggesting, or have I misunderstood? – Charles Rezk Oct 29 2009 at 18:47 I don't know if any Eilenberg-Steenrod (co)homology sits between these two example. The point is simply that sheaf theoretic singular (co)homology is of Eilenberg-Steenrod type and that it coincides with the usual one exactly for the spaces satisfying conditions 1)-4). This shows that for spaces which do not satisfy these properties, uniqueness of Eilenberg-Steenrod (co)homologies fails (a little issue with this story is that, for homology, you have to allow it takes its values in Pro(Ab); for cohomology, this should give genuine counter examples though). – Denis-Charles Cisinski Oct 30 2009 at 1:54 Another point: we can do all this for R-linear cohomologies, where R is any fixed commutative ring. The class of spaces a considered above then depends on R (hence we should expect some spaces to be good for some R, and bad for some R'). – Denis-Charles Cisinski Oct 30 2009 at 2:01

Clark Barwick suggested to me that this should be true for any Δ-generated space, and that this is a strictly larger class than spaces homotopy equivalent to a CW-complex. I haven't attempted to verify either of these statements.

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 A Delta-generated space is just a colimit of copies of I. So the long line is Delta-generated, and is not homotopy equivalent to a CW-complex. As an aside, in general topology, completely regular spaces are the category of choice, and they are the reflective subcategory generated by I. Delta-spaces are the coreflective subcategory generated by I. I think this is nice! – Mark Hovey Nov 12 2009 at 18:57