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I recently started learning rational homotopy theory, and found the claim on page 7 of this survey that there is a suitable model category structure in which the weak equivalences are the rational homotopy equivalences, the rationalization $X_{\mathbb{Q}}$ is a fibrant replacement of $X$. However, there was no detail on it.

I want to find a model category structure on (some full subcategory of) the category $\mathbf{Top}$ of topological spaces such that

  • Weak equivalences are rational homotopy equivalences.
  • CW complexes are cofibrants.
  • Rational spaces are fibrants.

How could we define cofibrations and fibrations on it?

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    $\begingroup$ That cannot be a model category for the trivial reason that it doesn’t have colimits. $\endgroup$
    – Zhen Lin
    Commented Oct 26 at 10:22
  • $\begingroup$ I really missed that part, thanks a lot. Then, is there no way to say that "rational homotopy theory is a homotopy theory where weak equivalences are rational homotopy equivalences"? $\endgroup$
    – Jun Heseŋ
    Commented Oct 26 at 10:42
  • $\begingroup$ In the entire set of notes you linked it appears that model categories are not referenced a single time. $\endgroup$ Commented Oct 26 at 10:47
  • $\begingroup$ Sorry for linked another note. I edited the link. $\endgroup$
    – Jun Heseŋ
    Commented Oct 26 at 10:53
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    $\begingroup$ I do not really see where they make the claim; what I see is that they claim there is a model structure on the category of all spaces, and then they make claims about the subcategory of the homotopy category consisting of rational and simply connected spaces. That is not to say that you can't talk about "the homotopy theory of rational simply connected spaces", but as Zhen Lin points out it cannot be made into a model category. $\endgroup$ Commented Oct 26 at 11:00

1 Answer 1

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Yes, there is such a model structure, at least if we are willing to pick a category of simply connected spaces that has limits and colimits.

The modern proof simply applies the existence theorem for left Bousfield localizations of combinatorial (like sSet) or cellular (like Top) model categories at a set of morphisms.

Simply connected spaces cannot be encoded by a left Bousfield localization of spaces, since the property of being simply connected is not stable under homotopy limits.

However, if we assume simple connectivity from the start, then it is easy to construct such a model structure. For example, start with the category of simplicial sets that have a single 0-simplex and single 1-simplex. This category has a model structure with the same weak equivalences as simplicial sets, and encodes pointed simply connected spaces. The left Bousfield localization of this model structure at the set of morphisms given by rationalizations of (pointed) spheres exists and encodes pointed simply connected rational spaces.

In this model structure, weak equivalences are precisely rational weak homotopy equivalences, fibrant objects are precisely rational Kan complexes, and cofibrant objects are precisely all objects (every simplicial set can be thought of as a CW-complex).

If one really wants topological spaces as the underlying object, we can do the following trick: encode simple connectivity as data instead of property. This amounts to specifying for every pair of points a specific choice of a path connecting them, and for every closed loop a specific choice of a disk filling out the loop. I haven't checked all the details, but I think the resulting category is complete and cocomplete and admits a model structure transferred from topological spaces along the forgetful functor, which can then be left Bousfield localized at rationalizations of spheres, obtaining a model category of pointed simply connected rational topological spaces.

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  • $\begingroup$ I suppose this model structure actually encodes pointed simply connected rational spaces. $\endgroup$ Commented Dec 2 at 12:24
  • $\begingroup$ @TimCampion: Thanks for spotting this, added “pointed” in a few places to make it more clear. $\endgroup$ Commented Dec 3 at 3:23

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