# Homology theory constructed in a homotopy-invariant way

Singular homology sends homotopic morphisms on equal morphisms and weakly equivalent spaces on isomorphic objects. So singular homology is in fact defined on the homotopy category of topological spaces.

But the usual definition of singular homology is on the category of topological spaces, and you can show that it is homotopy-invariant only after having defined it on the category of topological spaces.
For example, the definition uses the free abelian group on the underlying set of the space of singular $n$-simplexes, and taking the underlying set of a space do not make sense in the homotopy category.

I would like to have a construction of singular homology that can be entirely carried out in the homotopy category.

I was thinking of categorifying the usual construction:
Take the free spectrum ($(\infty,1)$ equivalent of abelian group?) on the space of singular n-simplexes, show that this is an "$(\infty,1)$ chain complex" and compute its "$(\infty,1)$ homology".

But I have the impression it will not work as is, because simplexes are only interesting as topological spaces, not as homotopy types.

Is there a way to construct singular homology (or in fact any homology theory) directly in the homotopy category without using the category of topological spaces?

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You could construct a $K(\mathbf{Z},n)$ directly and define $H^n(X)$ as homotopy classes of maps into it. But I guess this is not what you are looking for. –  Dan Petersen Oct 18 '11 at 13:12
Thank for your answers, I think my question was rather to define homology in a way that would work in any $(\infty,1)$-topos (seen as a generalization of the homotopy category), rather than try to characterize the homotopy category. And I think Eilenberg-MacLane spaces and smash products are always definable there, so Tyler's answer and Dan's comment (for cohomology) are what I was searching for. –  Guillaume Brunerie Oct 22 '11 at 9:36
@Guillaume, if you're looking for general ways to define "homology" that share some of the same properties you might look into either Andre-Quillen homology or Goodwillie calculus. –  Tyler Lawson Oct 22 '11 at 12:48
If you are happy to speak the language of $(\infty,1)$-categories, singular homology is the unique cocontinuous functor which sends the point to $\mathbf{Z}$. This characterization is also a construction of singular homology (which might be seen as a tiny baby version of the theory of Kan extensions in $(\infty,1)$-categories). –  Denis-Charles Cisinski Jan 27 '12 at 21:54
If you live in a based category, you might consider \textit{based} simplicies. What I mean by a based n-simplex is the standard n-simplex, with the vertices identified. Even though the n-simplices are not homotopically interesting, the based simplices are. They are all $K(\pi,1)''s$ where the group is a free group on the number of letters given by the dimension. –  Spice the Bird Jan 28 '12 at 1:31
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Throughout the following, I'll say "homotopy category" when I really mean the weak homotopy category.

For a space $X$, the homology of $X$ is canonically isomorphic to the reduced homology of $X_+$, which is $X$ with a disjoint basepoint. Therefore, it suffices to give a definition of the reduced homology of a based space.

The smash product $\wedge$ descends to a well-defined operation on the homotopy category of based topological spaces. For any $n \geq 0$, we have an object $K(\mathbb{Z},n)$ in the homotopy category, and weak equivalences $K(\mathbb{Z},n) \to \Omega K(\mathbb{Z},n+1)$ which are adjoint to maps $S^1 \wedge K(\mathbb{Z},n) \to K(\mathbb{Z},n+1)$. For any integer $m$, we can therefore define a directed system of sets $$[S^{m+k}, K(\mathbb{Z},k) \wedge X ] \to [S^{m+k + 1}, K(\mathbb{Z},k+1) \wedge X ] \to \cdots$$ The colimit $colim_k \pi_{m+k}( K(\mathbb{Z},k) \wedge X)$ is isomorphic to the $m$'th reduced homology group of $X$ in a canonical way.

This kind of definition produces generalized homology theories, and this all falls into the subject of stable homotopy theory.

The method you suggest of taking the free spectrum in this $\infty$-categorical sense will give a homology theory, but instead of producing the homology groups of $X$ it will produce the stable homotopy groups. The "free abelian topological group" on the topological space $X$ can be used instead, and shown to give a good notion on the homotopy category; this will produce the homology of $X$, as a result of the work of Dold and Thom.

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There are many good references for this sort of thing, but "Algebraic Topology from a Homotopical Viewpoint" by Aguilar, Gitler, and Prieto might be a good starting point. –  Greg Friedman Oct 18 '11 at 17:30
Do you know a reference for a construction of the "free abelian topological group" which happens directly in the homotopy category? –  Mike Shulman Oct 19 '11 at 0:24
Mike, no, not unless you maybe permit me to let the symmetric power functor take values in the homotopy category of $\Sigma_n$-spaces. However, I can't construct Eilenberg-Mac Lane spaces, enriched function objects, or the wedge product there either, though perhaps I could recognize them. I'm not sure that a definition of homology exists without some foreknowledge of properties of the homotopy category of spaces. If it did, why couldn't I use the same definition elsewhere? I think that the subject's development indicates that the homotopy category is not enough for most applications. –  Tyler Lawson Oct 19 '11 at 1:01
Using this definition, is it easy to compute the homology groups of spheres (for example)? –  Guillaume Brunerie Oct 22 '11 at 10:18
@Guillaume, it is "easy", but only in the sense that you can appeal to the Freudenthal suspension theorem. –  Tyler Lawson Oct 22 '11 at 12:44

I don't know what you are looking for, but here is my idea:

You could use the universal property of the homotopy category among "higher categories" as follows: For any "higher category" $C$ and any object $c$ of $C$, there is up to isomorphism one functor from the homotopy category of spaces - let's call it $H$ - to $C$ sending the point to $c$. Here, "higher categories" could mean homotopy categories of model categories, where the functors we allow are derived left Quillen functors, or (I guess) $\infty$-categories.

The derived category of $\mathbb Z$ is such a category, so we find a unique functor $F: H \rightarrow D(\mathbb Z)$ sending the point to the chain complex $U$ which has only $\mathbb Z$ in degree $0$. Now $D(\mathbb Z)$ is triangulated and we can define $H_{\star}(X) = [U, F(X)]_*$ for $X \in H$. It is easy to check that this is a homology theory and has the correct behaviour on the point. (This approach also has the advantage that the Künneth formula follows from abstract nonsense.)

To me, something like this seems to be the only way to avoid the mentioning of topological spaces or simplicial set, since the category $H$ either has to be given explicitly or characterised by some property as above. And introducing "higher category" structures also seems natural since you cannot properly define a homology theory on some random category; you need some additional structure such as cofiber and fiber sequences and suspension etc.

There is also the notion of an algebraic model category (and probably there is the same notion for $\infty$-categories), and our functor $F$ is the initial functor from $H$ to the homotopy category of an algebraic model category. In this way, one could view singular homology as the universal algebraic invariant of a homotopy type, but this is rather vague.

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This looks like a fancy way of stating the Eilenberg-Steenrod axioms and the uniqueness theorem for homology theories (on CW-complexes). I guess that the proof of the universal property, in particular the construction of the functor $F$, uses topological spaces or simplicial sets in an essential way. Or am I wrong? –  Johannes Ebert Oct 18 '11 at 16:47
You are right. I don't think one can give a satisfactory description of the homotopy category without at some point referring to spaces or simplicial sets or some other underlying structure; in this sense, the answer to the question is probably "no". I probably just read too much into the $\infinity$-categories appearing in the question at some point anyway, and the better answer is the one below. –  Fabian Lenhardt Oct 18 '11 at 18:28
I do like the perspective of a "universality" property of spaces as a way to construct derived functors out. –  Tyler Lawson Oct 19 '11 at 1:03
What do you mean by the derived category of $\mathbb{Z}$? Is it $\mathbb{Z}$ seen as a one-object category? –  Guillaume Brunerie Oct 22 '11 at 9:36

I think your intuition of founding algebraic topology through homotopical methods is sound, and I am pleased to report that this has been done in the two papers:

R. Brown and P.J. Higgins, On the algebra of cubes'', J. Pure Appl. Algebra 21 (1981) 233-260.

R. Brown and P.J. Higgins, Colimit theorems for relative homotopy groups'', J. Pure Appl. Algebra 22 (1981) 11-41.

In the space of less than 60 pages, and without using simplicial approximation or singular homology, these papers prove

1. the Brouwer Degree Theorem (the $n$-sphere $S^n$ is $(n-1)$-connected and the homotopy classes of maps of $S^n$ to itself are classified by an integer called the degree of the map);

2. the Relative Hurewicz Theorem, which is seen here as describing the morphism $$\pi_n(X,A,x) \to \pi_n(X \cup CA,CA,x) \cong \pi_n(X \cup CA,x)$$ when $(X,A)$ is $(n-1)$-connected, and so does not require the usual involvement of homology groups;

3. Whitehead's theorem (1949) that $\pi_2(X \cup \{e^2_{\lambda} \},X,x)$ is a free crossed $\pi_1(X,x)$-module.

The last theorem allows, for the usual identification of a square $\sigma$ to give the Klein bottle, to write the nonabelian formula:

$$\delta \sigma = a+b -a +b.$$ Whitehead's theorem is sometimes mentioned in texts but rarely proved. For more on the background to the theorem, see also http://ncatlab.org/nlab/show/free+crossed+module.

Actually one has here a method for direct calculation of some homotopy $2$-types, as crossed modules.

Historical background and intuitions for this method are given in an expository paper showing the origins of the idea of cubical higher homotopy groupoid. Not surprisingly, the method needs a number of new ideas, and departures from tradition, particularly the generalisation to higher dimensions of the fundamental groupoid and the Seifert-van Kampen Theorem.

The method is not to start with just a topological space but with a filtered space: i.e. a space $X$ and an increasing sequence of subspaces $X_n, n \geqslant 0$. This enables one to define a crossed complex $\Pi X_*$ using the fundamental groupoid of $X_1$ on the set $X_0$ of base points and for $n \geqslant 2$ the relative homotopy groups $\pi_n(X_n,X_{n-1},x), x \in X_0$, with the operations of $\pi_1$ and the standard boundary operations. The functor $\Pi$ satisfies a form of the Seifert-van Kampen Theorem, i.e. it can be computed on certain pushouts (and more generally) of filtered spaces. This gives the above results, and more. So one get new nonabelian calculations of second relative homotopy groups; and of higher relative homotopy groups as modules over a fundamental group, without using covering spaces.

However this result is not proved directly, but via a related (and not so trivial to establish!) construction of a cubical higher homotopy groupoid $\rho X_*$ of a filtered space. The first paper above establishes many key algebraic properties of these gadgets. These higher homotopy groupoids are strict structures, like the relative homotopy groups, and so a colimit type theorem allows for precise calculations.

The use of filtered spaces may be a surprise, though they are very common. However Grothendieck in Section 5 of "Esquisse d'un programme" argues that more structured concepts than a topological space are needed for the purposes of geometry: he likes stratifications. We use filtered spaces because in that context we can make these strict higher homotopy groupoids work, and express intuitions such as "algebraic inverses to subdivisions", for which simplicial or globular theories are not so well suited.

This work has been developed since those papers were published and some details of exposition modified: a long account is published as

R. Brown, P.J. Higgins, R. Sivera, Nonabelian algebraic topology: filtered spaces, crossed complexes, cubical homotopy groupoids, EMS Tracts in Mathematics Vol. 15, 703 pages. (August 2011).