Rational homotopy theory
In rational homotopy theory one considers topological spaces only up to maps that induce isomorphisms on rationalized homotopy groups (as opposed to genuine weak homotopy equivalences, which are those maps that induce isomorphism on the genuine homotopy groups.)
Every simply connected space is in this sense equivalent to a rational space: this is its rationalization.
Similarly one may consider “real-ification” by considering , etc.
Rationalization of a single space
A rationalization of a simply connected topological space is a continuous map where
is a simply connected rational space;
induces an isomorphism on rationalized homotopy groups:
or equivalently if induces an isomorphism on rational homology groups
Rationalization as a localization of /
In rational homotopy theory one considers the Quillen adjunction
between the model structure on dg-algebras and the standard model structure on simplicial sets, where is forming Sullivan differential forms:
Intrinsically this should model something like the (partially) left exact localization of an (∞,1)-category of ∞Grpd at those morphisms that are rational homotopy equivalences.
Below we review classical results that says that the left adjoint (infinity,1)-functor here indeed preserves at least homotopy pullbacks.
More generally, a setup by Bertrand Toen serves to provide a more comprehensive description of this situtation: see rational homotopy theory in an (infinity,1)-topos.
Preservation of homotopy pullbacks
This is effectively a restatement of a result that appears effectively below proposition 15.8 in HalperinThomas and is reproduced in some repackaged form as theorem 2.2 of He06. We recall the model category-theoretic context that allows to rephrase this result in the above form.
Let be the pullback diagram category.
The homotopy limit functor is the right derived functor for the Quillen adjunction (described in detail at homotopy Kan extension)
At model structure on functors it is discussed that composition with the Quillen pair induces a Quillen adjunction
We need to show that for every fibrant and cofibrant pullback diagram there exists a weak equivalence
here is a fibrant replacement of in .
Every object is cofibrant. It is fibrant if all three objects , and are fibrant and one of the two morphisms is a fibration. Let us assume without restriction of generality that it is the morphism that is a fibration. So we assume that and are three Kan complexes and that is a Kan fibration. Then sends to the ordinary pullback in , and so the left hand side of the above equivalence is
Recall that the Sullivan algebras are the cofibrant objects in , hence the fibrant objects of . Therefore a fibrant replacement of may be obtained by
first choosing a Sullivan model
then choosing factorizations in of the composites of this with and into cofibrations follows by weak equivalences.
The result is a diagram
that in exhibits a fibrant replacement of . The limit over that in is the colimit
in . So the statement to be proven is that there exists a weak equivalence
This is precisely the statement of that quoted result He, theorem 2.2.
check the following
Rationalization preserves homotopy pullbacks of objects of finite type.
The theory of Sullivan models asserts that rationalization of a space (a simplicial set ) is the derived unit of the derived adjunction , namely that the rationalization is modeled by applied to a Sullivan model for .
Being a Quillen right adjoint, the right derived functor of of course preserves homotopy limits. Hence the composite preserves homotopy pullbacks between objects of finite type.
Rationalization of spectra
On spectra, rationalization is a smashing localization, given by smash product with the Eilenberg-MacLane spectrum . (e.g. Bauer 11, example 1.7 (4)).
For more see at rational stable homotopy theory.
Around definition 1.4 in