model category

for ∞-groupoids

# Contents

## Idea

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.

## Definition

For $C$ and $D$ two model categories, a pair $(L,R)$

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

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

• $L$ preserves cofibrations and acyclic cofibrations;

• $R$ preserves fibrations and acyclic fibrations;

• $L$ preserves cofibrations and $R$ preserves fibrations;

• $L$ preserves acyclic cofibrations and $R$ 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.

## Properties

### General

###### Proposition

It follows from the definition that

• the left adjoint $L$ preserves weak equivalences between cofibrant objects;

• the right adjoint $R$ preserves weak equivalences between fibrant objects.

###### Proof

To show this for instance for $R$, 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 $R$ and hence by 2-out-of-3 the claim follows.

For $L$ we apply the formally dual argument.

### Behaviour under localization

###### Proposition

If

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

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

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

This appears as (Hirschhorn, prop. 3.3.18)

### Of $sSet$-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.

###### Proposition

Let $C$ and $D$ be simplicial model categories and let

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

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

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

On the decategorified level of the homotopy categories these are the total left and right derived functors, respectively, of $L$ and $R$.

###### Proof

This is proposition 5.2.4.6 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).

###### Proposition

If $C$ and $D$ are simplicial model categories and $D$ is a left proper model category, then an sSet-enriched adjunction

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

is a Quillen adjunction already if $L$ preserves cofibrations and $R$ just fibrant objects.

This appears as HTT, cor. A.3.7.2.

See simplicial Quillen adjunction for more details.

## References

See the references at model category. For instance

Revised on November 7, 2014 23:19:02 by David Roberts (58.179.239.189)