# Exact sequences in homotopy categories

I am not really familiar with homotopical category theory, so please forgive me if I make rude mistakes. I know quite a bit of common category theory, as well as familiar with algebraic topology.

How can one construct exact sequences abstractly in the framework of homotopical categories? Consider any model you like (topologically or simplicially enriched, or $(\infty,1)$-categories). nLab teaches us that to construct exact sequences in topology one needs to consider fiber sequences and homotopy limits/colimits. A fiber sequence $F\to E \to B$ is a homotopy pullback

\begin{array}{ccc} F & \rightarrow & E\\\ \downarrow & & \downarrow\\\ \ast & \rightarrow & B\\\ \end{array}

The loop space $\Omega X$ of $X$ is defined as a homotopy pullback

\begin{array}{ccc} \Omega X & \rightarrow & \ast \\\ \downarrow & & \downarrow\\\ \ast & \rightarrow & X\\\ \end{array}

Dually, one can define cofiber sequences and suspensions $\Sigma X$. We get a standard adjunction $\Sigma \vdash \Omega$. Given a fiber sequence, one can have a long fiber sequence $\dots\to\Omega E \to \Omega B \to F \to E \to B$. To begin with, consider a homotopy pullback rectangle

\begin{array}{ccc} \Omega B & \rightarrow & \ast \\\ \downarrow & & \downarrow\\\ F & \rightarrow & E\\\ \downarrow & & \downarrow\\\ \ast & \rightarrow & B\\\ \end{array}

Standard diagram chasing and (homotopic) universality arguments show that we get a fiber sequence $\Omega \to F \to E$ etc. Now if we apply the functor $\pi_0 \simeq \pi(\ast,-)$, we get a sequence $$\dots \to \pi_0(\Omega E) \to \pi_0(\Omega B) \to \pi_0(F) \to \dots$$

Using the fact that $\pi_0$ preserves fiber sequences (because it is representable) and $\pi_0(\Omega^i X) \simeq \pi_i (X)$ we get a classical long exact sequence of homotopy groups for a fibration.

Now things get trickier if we consider homotopy pushouts instead of pullbacks. If we have a cover $(U,V)$ of space $X$, then we have a homotopy pushout diagram

\begin{array}{ccc} U\cap V & \rightarrow & U \\\ \downarrow & & \downarrow\\\ V & \rightarrow & X\\\ \end{array}

Standard arguments allow to replace this diagram with cofiber sequence $U \vee V \to X \to \Sigma (U\cap V)$. In theory it should give Seifert-van Kampen theorem and Mayer-Vietoris sequences in homology and cohomology in a similar way. In practice, I don't understand how can this happen. To get Seifert-van Kampen theorem, one should apply $\pi_1\simeq \pi_0 \circ \Omega$, but $\Omega$ fails to preserve cofiber sequences. Dold-Thom theorem tells us that $H_i = \pi_i \circ S$, where $S$ is the topological abelianization functor. We're fine with $S$, because it's a left adjoint to forgetful functor, but we have the same problem with $\pi_i$. It doesn't preserve cofiber sequences. Even if it did, we would have terms like $\pi_i (S \Sigma X)=H_i (\Sigma X) = H_{i-1}(X)$ for $i>0$, and that is nothing like an ascending sequence of homology groups. Even cohomology isn't fine: we can take the space of maps to $K(\mathbb{Z},1)$ and it turns cofiber sequences into fiber sequences, but we still have a problem with grading, like in case of homology.

So my question is: how can we deal with these difficulties and get the mentioned theorems? I didn't have any success till now in finding references on homotopy limits/colimits technique, although many sources mention them and deal with technical properties, like their existence in model categories. Any references would be greatly appreciated!

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MathOverflow does not work with XyMatrix. If you want commutative diagrams, try using arrays. –  S. Carnahan Feb 29 '12 at 7:57
Somewhy all arrays become written in a line, instead of a table. \begin{array} dummy & dummy \\ also & dummy \end{array}, right? MO's TeX is weird. –  Anton Fetisov Feb 29 '12 at 19:03

I don't think the Seifert-van Kampen theorem follows from these kinds of considerations. Rather, it is the statement that the fundamental groupoid functor $\tau_{\leq 1}$ preserves homotopy colimits. That's because it is left adjoint to the inclusion of 1-groupoids in $\infty$-groupoids (a generalization to any $\infty$-category is in Higher Topos Theory, Prop. 5.5.6.18).

[Removed the part on cohomology because it got me confused!]

Added: I attempted to explain the long exact sequences in cohomology without using spectra, but that was wrong. Here's a correct explanation. The reduced cohomology of a pointed space $X$ depends only on its stabilization $\Sigma^\infty X$: it is given by $H^n(X;A)=[\Sigma^\infty X,\Sigma^n HA]$ where $HA$ is an infinite delooping of $K(A,0)$. The functor $\Sigma^\infty$ preserves cofiber sequences (being left adjoint). Now if you have a cofiber sequence in a stable category, you get long exact sequences of abelian groups when you apply functors like $[E,-]$ or $[-,E]$.

Added later: My original answer was correct, but like I said I got confused… Here it is again. Let $A\to B\to C$ be a cofiber sequence of pointed spaces. As you say in your question, you get a fiber sequence of mapping spaces

$Map(C,X)\to Map(B,X)\to Map(A,X)$

for any $X$, because $Map$ transforms homotopy colimits in its first variable into homotopy limits. In its second variable it preserves homotopy limits, so $\Omega Map(A,X)=Map(A,\Omega X)$. Applying to $X=K(G,n)$ gives you the usual long exact sequence in cohomology, but only from $H^0$ to $H^n$.

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As it was already pointed out, those statements will not follow from such general principles. What you would like to conclude is that homotopy pushouts are (or to certain degree behave like) homotopy pullbacks. Such a phenomenon is called excision. In the language of $\infty$-categories you can define a functor $\mathcal{C} \to \mathcal{D}$ to be excisive if it sends homotopy pushouts to homotopy pullbacks. However, being excisive is rather strong condition and proving that some functor satisfies it usually takes some work. In particular I don't think there is some abstract way around the usual point-set arguments used to prove Seifert--van Kampen theorem or various excision theorems (i.e. in homotopy or singular homology).

There are lots of generalizations of this notion that consider higher connectivity, functors of many variables etc. Those ideas give rise to Goodwillie calculus. Jacob Lurie has recently updated his Higher Algebra which now containes a full chapter developing Goodwillie calculus in the language of $\infty$-categories. Some more basic properties of excisive functors are discussed in section 1.4.

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I hope someone will relate that work to our papers (RB and J.-L. LODAY), Excision homotopique en basse dimension'', {\em C.R. Acad. Sci. Paris S\'er.} I 298 (1984) 353-356. (RB and J.-L. LODAY), Homotopical excision, and Hurewicz theorems, for $n$-cubes of spaces'', Proc. London Math. Soc. (3) 54 (1987) 176-192. Excision usually involves pairs. The latter paper has some cubical dodges to generalise excision to a space with $n$ subspaces, or more generally an $n$-cube of spaces. –  Ronnie Brown Mar 1 '12 at 11:19

I don't think the Seifert-van Kampen Theorem is about homotopy colimits and $\infty$-groupoids, but that it allows precise, or, if you like, strict, calculations of some homotopy 1-types as colimits. Thus it is about the functor $\pi_1$ from (spaces with sets of base points) to (strict) groupoids. This links it more to combinatorial group(oid) theory, rather than higher category theory. The loop space is a functor from spaces with but one base point.

Similarly, the higher homotopy Siefert-van Kampen theorems are about functors from spaces with structure (filtrations or $n$-cubes of spaces) to certain strict higher groupoids, or variants, and state that they preserve certain (strict) colimits. This allows for some precise colimit calculations in homotopy theory, avoiding where applicable the necessity to find ways of calculating extensions in exact sequences, and in fact calculates some homotopy $n$-types, even allowing some computer calculations.

Also the proofs of the SvKTs do not follow methods of abstract homotopy theory. Is that a Good Thing or a Bad Thing?

For references, see articles, etc., on my web site.

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I think the result I referenced from HTT in my answer is a strict (and vast) generalization of SvK (and probably of its higher-n versions, modulo the computation of homotopy colimits of $n$-groupoids). A pushout of groupoids along inclusions is a homotopy pushout, so you directly recover the usual SvK, but it also works for open covers with more patches, quotients by free group actions, etc. As to your last sentence, I find the proof in HTT pretty abstract :) –  Marc Hoyois Feb 29 '12 at 12:47
I find the word "probably" just a bit airy-fairy, or maybe just a statement of a research project! Start with the 2-dimensional SvKT, which computes precisely some nonabelian second relative homotopy groups as crossed modules. Examples are in (RB and C.D.WENSLEY), `Computation and homotopical applications of induced crossed modules', J. Symbolic Computation 35 (2003) 59-72, also in the book "Nonabelian algebraic topology". And many precise computations for 3-types are in the literature on the nonabelian tensor product of groups, using the algebraic structures underlying the geometry. –  Ronnie Brown Feb 29 '12 at 21:01
You're right, I should have said "maybe". I have absolutely zero knowledge of $\geq 2$ van Kampen theorems, so that was just my mathematical optimism speaking. Certainly the fact that $\tau_{\leq n}$ preserves homotopy colimits is a natural generalization of the ususal van Kampen from $1$ to $n$, but I don't know to what extent it matches so-called higher van Kampen theorems. My uneducated guess is that it is the "homotopical essence" of higher SvK theorems, and that the rest is getting your hands on $n$-groupoids. –  Marc Hoyois Feb 29 '12 at 22:58
We got going on higher SvKTs when we realised a 2-d version could profitably involve a double groupoid functor from pointed (or many-pointed) pairs of spaces to double groupoids, so yielding a SvKT for second relative homotopy groups as crossed modules, moving the classical proof up one dimension. So far, nobody has been able to deduce this theorem from some "$\tau$ is a left adjoint" principle! This result is also related to excision, but involving actions. The main point was getting hold of functors which could model algebraically the geometry of the proof! The rest followed. –  Ronnie Brown Mar 1 '12 at 11:09