Quillen adjunction


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Quillen adjunctions are one convenient notion of morphism between model categories. They present adjoint (∞,1)-functors between the (∞,1)-categories presented by the model categories.


For CC and DD two model categories, a pair (L,R)(L,R)

(LR):CLRD (L \dashv R) : C \stackrel{\overset{R}{\leftarrow}}{\underset{L}{\to}} D

of adjoint functors (with LL left adjoint) is a Quillen adjunction if the following equivalent conditions are satisfied:

  • LL preserves cofibrations and acyclic cofibrations;

  • RR preserves fibrations and acyclic fibrations;

  • LL preserves cofibrations and RR preserves fibrations;

  • LL preserves acyclic cofibrations and RR preserves acyclic fibrations.

Quillen adjunctions that are analogous to an equivalence of categories are called Quillen equivalences.

In an enriched model category one speaks of enriched Quillen adjunction.




It follows from the definition that

  • the left adjoint LL preserves weak equivalences between cofibrant objects;

  • the right adjoint RR preserves weak equivalences between fibrant objects.


To show this for instance for RR, we may argue as in a category of fibrant objects and apply the factorization lemma which shows that every weak equivalence between fibrant objects may be factored, up to homotopy, as a span of acyclic fibrations.

These weak equivalences are preserved by RR and hence by 2-out-of-3 the claim follows.

For LL we apply the formally dual argument.

Behaviour under localization



(LR):CLRD (L \dashv R) : C \stackrel{\overset{R}{\leftarrow}}{\underset{L}{\to}} D

is a Quillen adjunction, SMor(D)S \subset Mor(D) is a set of morphisms such that the left Bousfield localization of DD at SS exists, and such that the derived image 𝕃L(S)\mathbb{L}L(S) of SS lands in the weak equivalences of CC, then the Quillen adjunction descends to the localization D SD_S

(LR):CLRD S. (L \dashv R) : C \stackrel{\overset{R}{\leftarrow}}{\underset{L}{\to}} D_S \,.

This appears as (Hirschhorn, prop. 3.3.18)

Of sSetsSet-enriched adjunctions

Of particular interest are SSet-enriched adjunctions between simplicial model categories: simplicial Quillen adjunctions.

These present adjoint (∞,1)-functors, as the first proposition below asserts.


Let CC and DD be simplicial model categories and let

(LR):CLRD (L \dashv R) : C \stackrel{\overset{R}{\leftarrow}}{\underset{L}{\to}} D

be an sSet-enriched adjunction whose underlying ordinary adjunction is a Quillen adjunction. Let C C^\circ and D D^\circ be the (∞,1)-categories presented by CC and DD (the Kan complex-enriched full sSet-subcategories on fibrant-cofibrant objects). Then the Quillen adjunction lifts to a pair of adjoint (∞,1)-functors

(𝕃):C D . (\mathbb{L} \dashv \mathbb{R}) : C^\circ \stackrel{\leftarrow}{\to} D^{\circ} \,.

On the decategorified level of the homotopy categories these are the total left and right derived functors, respectively, of LL and RR.


This is proposition in HTT.

The following proposition states conditions under which a Quillen adjunction may be detected already from knowing of the right adjoint only that it preserves fibrant objects (instead of all fibrations).


If CC and DD are simplicial model categories and DD is a left proper model category, then an sSet-enriched adjunction

(LR):CD (L \dashv R) : C \stackrel{\leftarrow}{\to} D

is a Quillen adjunction already if LL preserves cofibrations and RR just fibrant objects.

This appears as HTT, cor. A.3.7.2.

See simplicial Quillen adjunction for more details.


See the references at model category. For instance

Revised on November 7, 2014 23:19:02 by David Roberts (