7
$\begingroup$

Suppose I have a functor $$ X_\bullet: I \to \text{Spaces} $$ where $I$ is a small filtered category.

It seems to be a "folk theorem" that the homomorphism $$ \underset{\alpha\in I}{\text{colim }} H_\ast(X_\alpha) \to H_\ast(\underset{\alpha\in I}{\text{hocolim }} X_\alpha) $$ is an isomorphism, where $H_\ast$ denotes singular homology.

Is there a reference for this?

(Preferably standard?)

Improve this question
$\endgroup$
4
  • 4
    $\begingroup$ I think an argument which is much simpler than the answer by A.S. can be given as follows. Since the homotopy colimit is indexed over the filtered category, it agrees with ordinary colimit, c.f. mathoverflow.net/questions/235526/… and the homology now commutes with filtered colimit. $\endgroup$
    – user43326
    Commented Jul 2, 2021 at 18:22
  • 1
    $\begingroup$ You could argue that, first, the singular chains functor from Spaces to Chain Complexes preserves filtered homotopy colimits because of compactness (in fact it preserves all homotopy colimits, but you don't need this), and second, homology preserves filtered homotopy colimit as a functor from chain complexes to Ab groups, which is an elementary exercise. Alternatively you can argue that homology of $X$ is the homotopy of $H\mathbb Z \wedge X$. The functor $H\mathbb Z \wedge -$ preserves arbitrary homotopy colimits, and homotopy groups preserve filtered homotopy colimits by compactness. $\endgroup$
    – Gregory Arone
    Commented Jul 4, 2021 at 8:08
  • 1
    $\begingroup$ @GregoryArone I am looking for a reference, not a proof. $\endgroup$
    – John Klein
    Commented Jul 4, 2021 at 18:04
  • 1
    $\begingroup$ @user43326 I don't think that is true: $\text{hocolim} X_\alpha$ needn't coincide with $\text{colim} X_\alpha$ in this case. Maybe you wish to work in Simplicial Sets? If so, what is the reference there for why homology commutes with filtered colimits? Again, I am asking for a reference. I know how to prove the statement. $\endgroup$
    – John Klein
    Commented Jul 4, 2021 at 18:11

1 Answer 1

Reset to default
6
$\begingroup$

I think the usual reference here is chapter XII section 5 of the Bousfield-Kan book "Homotopy limits, completions, and localizations," where they build a spectral sequence $E_2^{*,*} \cong L_*colim_i H_*(X_i) \Rightarrow H_*(hocolim_i X_i)$. The rest of the argument is not explicit in the book (but it's very straightforward so I think it's still correct to cite their book for your result): since your diagram is filtered, the derived functors $L_*colim_i H_*(X_i)$ vanish in positive degrees, so the spectral sequence collapses to the $L_0colim_i H_*(X_i)$ line, yielding your isomorphism.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ Yes, I was aware of that it is easily deducible from the spectral sequence, but I am nonetheless surprised that it doesn't seem to be explicitly stated anywhere in the standard literature $\endgroup$
    – John Klein
    Commented Jul 2, 2021 at 17:18
  • 1
    $\begingroup$ Maybe it is stated somewhere, but nowhere that I'm aware of. The reason that I think the Bousfield-Kan book is the right reference is that people often regard that book (from 1972) as having set up the first rigorous and general theory of homotopy (co)limits in an unstable setting. If that is accurate, then earlier references won't have the isomorphism you want, since a general theory of homotopy colimits of filtered diagrams of spaces wasn't in place before that book. But I do not know if there a similarly classical reference which is more explicit about your isomorphism. $\endgroup$
    – user164898
    Commented Jul 2, 2021 at 17:29

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .