# nLab model category

Contents

model category

## Model structures

for ∞-groupoids

### for $(\infty,1)$-sheaves / $\infty$-stacks

#### $(\infty,1)$-Category theory

(∞,1)-category theory

# Contents

## Idea

A model category (sometimes called a Quillen model category or a closed model category, but not related to “closed category”) is a context for doing homotopy theory. Quillen developed the definition of a model category to formalize the similarities between homotopy theory and homological algebra: the key examples which motivated his definition were the category of topological spaces, the category of simplicial sets, and the category of chain complexes.

So, what is a model category? For starters, it is a category equipped with three classes of morphisms, each closed under composition and called weak equivalences, fibrations and cofibrations:

A bit more technically: we can define an (∞,1)-category starting from any category with weak equivalences. The idea is that this (∞,1)-category keeps track of objects in our original category, morphisms between objects, homotopies between morphisms, homotopies between homotopies, and so on, ad infinitum. However, the extra structure of a model category makes it easier to work with this (∞,1)-category. We can obtain this (∞,1)-category in various ways, such as simplicial localization of the underlying category with weak equivalences, or (if the model category is simplical) the homotopy coherent nerve of the simplicial subcategory $M_{cf}\subset M$ of cofibrant-fibrant objects. We say this (∞,1)-category is presented (or modeled) by the model category, and that the objects of the model category are models for the objects of this $(\infty,1)$-category. Not every (∞,1)-category is obtained in this way (otherways it would necessarily have all small homotopy limits and homotopy colimits).

In this sense model categories are ‘models for homotopy theory’ or ‘categories of models for homotopy theory’. (The latter sense was the one intended by Quillen, but the former is also a useful way to think.)

Recall that the idea of categories with weak equivalences is to work just with 1-morphisms instead of with n-morphisms for all $n$, but to carry around extra information to remember which 1-morphisms are really equivalences in the full (∞,1)-category, i.e. isomorphisms in the corresponding homotopy category.

In a model category the data of weak equivalences is accompanied by further auxiliary data that helps to compute the (∞,1)-categorical hom-space, the homotopy category and derived functors. See homotopy category of a model category for more on that.

If the model category happens to be a combinatorial simplicial model category $\mathbf{A}$ it presents the category $\mathbf{A}^\circ$ in the form of a simplicially enriched category given by the full SSet-enriched subcategory on objects that are both fibrant and cofibrant.

## Definition

The following is a somewhat terse account. For a more detailed exposition see at Introduction to Homotopy Theory the section Abstract homotopy theory.

###### Definition

A model structure on a category $\mathcal{C}$ is a choice of three distinguished classes of morphisms

• cofibrations $Cof \subset Mor(\mathcal{C})$,

• fibrations $Fib \subset Mor(\mathcal{C})$,

• weak equivalences $W \subset Mor(\mathcal{C})$

satisfying the following conditions:

1. $W$ makes $\mathcal{C}$ into a category with weak equivalences,

(meaning that it contains all isomorphisms and is closed under two-out-of-three: given a composable pair of morphisms $f,g$, if two out of the three morphisms $f, g, g f$ are in $W$, so is the third);

2. $(Cof, Fib \cap W)$ and $(Cof \cap W, Fib)$ are two weak factorization systems on $\mathcal{C}$.

###### Definition

A model category is a complete and cocomplete category $\mathcal{C}$ equipped with a model structure according to def. .

This equivalent version of the definition was observed in (Joyal, def. E.1.2), highlighted in (Riehl 09). This definition already implies all the closure conditions on classes of morphisms which other definitions in the literature explicitly ask for, see below.

###### Definition

(terminology)

• The morphisms in $W \cap Fib$ (the fibrations that are also weak equivalences) are called trivial fibrations or acyclic fibrations

• The morphisms in $W \cap Cof$ (the cofibrations that are also weak equivalences) are called trivial cofibrations or acyclic cofibrations.

• An object is called cofibrant if the unique morphism $\emptyset \to X$ from the initial object is a cofibration

• An object is called fibrant if the unique morphism $X\to *$ to the terminal object is a fibration.

###### Remark

Often, the fibrant and cofibrant objects are the ones one is “really” interested in, but the category consisting only of these is not well-behaved (as a 1-category). The factorizations supply fibrant and cofibrant replacement functors which allow us to treat any object of the model category as a ‘model’ for its fibrant-cofibrant replacement.

## Variants

### Slight variations on the axioms

Quillen’s original definition required only finite limits and finite colimits, which are enough for the basic constructions. Colimits of larger cardinality are sometimes required for the small object argument, however. This change was popularized by Dwyer, Hirschhorn, Kan, and Smith in their book Homotopy Limit Functors on Model Categories and Homotopical Categories

Robert W. Thomason proposed to require that the factorizations given by (ii) are actually functorial factorization systems,

resulting in the notion of a Thomason model category. Mark Hovey later included the data of such a functorial factorization (and not just its existence) into his definition of a model category. In practice, Quillen’s small object argument means that many model categories can be made to have functorial factorizations. (But not all: an example of a model category with non-functorial factorizations can be found in Isaksen 2001.)

### Enhancements of the axioms

There are several extra conditions that strengthen the notion of a model category:

### Weaker axiom systems

There are several notions of category with weak equivalences with similar but less structure than a full model category.

• A category of fibrant objects has a notion of just weak equivalences and fibrations, none of cofibrations. As the name implies, all of its objects are fibrant; the canonical example is the subcategory of fibrant objects in a model category.

• A Waldhausen category dually has a notion of weak equivalences and cofibrations, and all of its objects are cofibrant.

• There is also a slight variant of the full notion of model category by Thomason that is designed to make the global model structure on functors more naturally accessible: this is the notion of Thomason model category.

• Semimodel categories? relax some of the conditions on lifting properties.

• Weak model categories? relax these conditions even further.

• Premodel categories? are an even weaker notion that allows

for a nice 2-categorical? treatment.

## Properties

### Closure of morphism classes under retracts

As a consequence of the definition, the classes $Cof, Fib$, and $W$ are all closed under retracts in the arrow category $Arr C$ and under composition and contain the isomorphisms of $C$.

For $Cof$ and $Fib$ and $W \cap Cof$ and $W \cap Fib$ this and further closure properties are discussed in detail at weak factorization system in the section Closure properties.

In the presence of functorial factorizations, it follows immediately that also $W$ is closed under retracts: for any retract diagram may then be funtorially factored with the middle morphism factored through $W \cap Cof$ followed by $W \cap Fib$, and so the statement follows from the above closure of these classes under retracts.

Without assuming functorial factorization the statement still holds:

###### Proposition

Given a model category in the sense of def. , then its class of weak equivalences is closed under forming retracts (in the arrow category).

(Joyal, prop. E.1.3), highlighted in (Riehl 09)

###### Proof

Let

$\array{ id \colon & A &\longrightarrow& X &\longrightarrow& A \\ & {}^{\mathllap{f}} \downarrow && \downarrow^{\mathrlap{w}} && \downarrow^{\mathrlap{f}} \\ id \colon & B &\longrightarrow& Y &\longrightarrow& B }$

be a commuting diagram with $w \in W$ a weak equivalence. We need to show that then also $f \in W$.

First consider the case that $f \in Fib$.

In this case, factor $w$ as a cofibration followed by an acyclic fibration. Since $w \in W$ and by two-out-of-three this is even a factorization through an acyclic cofibration followed by an acyclic fibration. Hence we obtain the commuting diagram

$\array{ id \colon & A &\longrightarrow& X &\overset{\phantom{AAAA}}{\longrightarrow}& A \\ & {}^{\mathllap{id}}\downarrow && \downarrow^{\mathrlap{\in W \cap Cof}} && \downarrow^{\mathrlap{id}} \\ id \colon & A' &\overset{s}{\longrightarrow}& X' &\overset{\phantom{AA}t\phantom{AA}}{\longrightarrow}& A' \\ & {}^{\mathllap{f}}_{\mathllap{\in Fib}} \downarrow && \downarrow^{\mathrlap{\in W \cap Fib}} && \downarrow^{\mathrlap{f}}_{\mathrlap{\in Fib}} \\ id \colon & B &\longrightarrow& Y &\underset{\phantom{AAAA}}{\longrightarrow}& B } \,,$

where $s$ is uniquely defined and where $t$ is any lift of the top middle vertical acyclic cofibration against $f$. This now exhibits $f$ as a retract of an acyclic fibration. These are closed under retract by this prop..

Now consider the general case. Factor $f$ as an acyclic cofibration followed by a fibration and form the pushout in the top left square of the following diagram

$\array{ id \colon & A &\longrightarrow& X &\overset{\phantom{AAAA}}{\longrightarrow}& A \\ & {}^{\mathllap{\in W \cap Cof}}\downarrow &(po)& \downarrow^{\mathrlap{\in W \cap Cof}} && \downarrow^{\mathrlap{\in W \cap Cof}} \\ id \colon & A' &\overset{}{\longrightarrow}& X' &\overset{\phantom{AA}\phantom{AA}}{\longrightarrow}& A' \\ & {}^{\mathllap{\in Fib}} \downarrow && \downarrow^{\mathrlap{\in W }} && \downarrow^{\mathrlap{\in Fib}} \\ id \colon & B &\longrightarrow& Y &\underset{\phantom{AAAA}}{\longrightarrow}& B } \,,$

where the other three squares are induced by the universal property of the pushout, as is the identification of the middle horizontal composite as the identity on $A'$. Since acyclic cofibrations are closed under forming pushouts by this prop., the top middle vertical morphism is now an acyclic fibration, and hence by assumption and by two-out-of-three so is the middle bottom vertical morphism.

Thus the previous case now gives that the bottom left vertical morphism is a weak equivalence, and hence the total left vertical composite is.

### Redundancy in the defining factorization systems

It is clear that:

###### Remark

Given a model category structure, any two of the three classes of special morphisms (cofibrations, fibrations, weak equivalences) determine the third:

• given $W$ and $C$, we have $F = RLP(W \cap C)$;

• given $W$ and $F$, we have $C = LLP(W \cap F)$;

• given $C$ and $F$, we find $W$ as the class of morphisms which factor into a morphism in $LLP(F)$ followed by a morphism in $RLP(C)$.

(Here $RLP(S)$ denotes the class of morphisms with the right lifting property against $S$ and $LLP(S)$ denotes the class of morphisms with the left lifting property against $S$.)

But, in fact, already the cofibrations and the fibrant objects determine the model structure.

###### Proposition

A model structure $(C,W,F)$ on a category $\mathcal{C}$ is determined by its class of cofibrations and its class of fibrant objects.

This statement appears for instance as (Joyal, prop. E.1.10)

###### Proof

Let $\mathcal{E}$ with $C,F,W \subset Mor(\mathcal{E})$ be a model category.

By remark it is sufficient to show that the cofibrations and the fibrant objects determine the class of weak equivalences. Moreover, these are already determined by the weak equivalences between cofibrant objects, because for $u : A \to B$ any morphism, functorial cofibrant replacement $\emptyset \hookrightarrow \hat A \stackrel{\simeq}{\to} A$ and $\emptyset \hookrightarrow \hat B \stackrel{\simeq}{\to} B$ with 2-out-of-3 implies that $u$ is a weak equivalence precisely if $\hat u : \hat A \to \hat B$ is.

By the nature of the homotopy category $Ho$ of $\mathcal{E}$ and by the Yoneda lemma, a morphism $\hat u : \hat A \to \hat B$ between cofibrant objects is a weak equivalence precisely if for every fibrant object $X$ the map

$Ho(\hat u, X) : Ho(\hat B, X) \to Ho(\hat A, X)$

is an isomorphism, namely a bijection of sets. The equivalence relation that defines $Ho(\hat A,X)$ may be taken to be given by left homotopy induced by cylinder objects, which in turn are obtained by factoring codiagonals into cofibrations followed by acyclic fibrations. So all this is determined already by the class of cofibrations, and hence weak equivalences are determined by the cofibrations and the fibrant objects.

### Opposite model structure

###### Proposition

If a category $C$ carries a model category structure, then the opposite category $C^{op}$ carries the opposite model structure:

its weak equivalences are those morphisms whose dual was a weak equivalence in $C$, its fibrations are those morphisms that were cofibrations in $C$ and similarly for its cofibrations.

See at

## Examples

Every category with limits and colimits carries the trivial model structure whose weak equivalences are the isomorphisms and all morphisms are cofibrations and fibrations.

### Classical model structures

The archetypical model structures are the

and the

These model categories are Quillen equivalent and encapsulate much of “classical” homotopy theory. From a higher-categorical viewpoint, they can be regarded as models for ∞-groupoids (in terms of CW complexes or Kan complexes, respectively).

The passage to stable homotopy theory is given by model structures on spectra built out of either of these two classical model structures. See at Model categories of diagram spectra for a unified treatment.

Accordingly homological algebra with its derived categories and derived functors (which may be thought of as a sub-topic of stable homotopy theory via the stable Dold-Kan correspondence) is reflected by

In fact, the original definition of model categories in Quillen1967 was motivated by the analogy between constructions in homotopy theory and homological algebra.

### Categorical model structures

Of interest to category theorists is that many notions of higher categories come equipped with model structures, witnessing the fact that when retaining only invertible transfors between $n$-categories they should form an $(\infty,1)$-category. Many of these are called

Model categories have successfully been used to compare many different notions of (∞,1)-category. The following definitions of $(\infty,1)$-category all form Quillen equivalent model categories:

There are related model structures for enriched higher categories:

Other “higher categorical structures” can also be expected to form model categories, such as the

which generalizes the Joyal model structure from (∞,1)-categories to (∞,1)-operads.

There is also another class of model structures on categorical structures, often called Thomason model structures (not to be confused with the notion of “Thomason model category”). In the “categorical” or “canonical” model structures, the weak equivalences are the categorical equivalences, but in the Thomason model structures, the weak equivalences are those that induce weak homotopy equivalences of nerves. Thomason model structures are known to exist on 1-categories and 2-categories, at least, and are generally Quillen equivalent to the Quillen model structure on topological spaces and thus (via the singular simplicial complex and geometric realization adjunction) to the and Quillen model structure on simplicial sets.

### Parametrized model structures

The parameterized version of the model structure on simplicial sets is a

which serves as a model for ∞-stack (∞,1)-toposes (for hypercomplete (∞,1)-toposes, more precisely).

### Functor and localized model structures

Many model structures, including those for complete Segal spaces, simplicial presheaves, and diagram spectra, are constructed by starting with a model structure on a functor category, such as a

and applying a general technique called Bousfield localization which forces a certain class of morphisms to become weak equivalences. It can also be thought of as forcing a certain class of objects to become fibrant.

### Limit and colimit model structures

Model structures can be induced on certain (usually lax) limits and colimits of diagrams of model categories.

Algebraic model structures: Quillen model structures, mainly on locally presentable categories, and their constituent categories with weak equivalences and weak factorization systems, that can be equipped with further algebraic structure and “freely generated” by small data.

structuresmall-set-generatedsmall-category-generatedalgebraicized
weak factorization systemcombinatorial wfsaccessible wfsalgebraic wfs
model categorycombinatorial model categoryaccessible model categoryalgebraic model category
construction methodsmall object argumentsame as $\to$algebraic small object argument

## References

The concept of a model category originates in Chapter I, Axiomatic homotopy theory, of

#### Coverage table for major sources

TopicQuillenHoveyHirschhornDHKSMay-PontoRiehlLurie
combinatorial model categoriesnonononono*yesyes
monoidal model categoriesnoyesnonoyesyesyes
enriched model categoriesnonononoyesyesyes
homotopy colimitsnonoyesyesno*yesyes
Bousfield localizationsnonoyesnoyesnoyes
transferred model structuresyesnoyesnonoyesyes
Reedy model structuresnoyesyesyesnoyesyes

#### Book chapters

For yet another introduction to model categories, with an eye towards their use as presentations of $(\infty,1)$-categories see

#### Other sources

An account is in

and appendix E of

The version of the definition in (Joyal) is also highlighted in

An introductory survey of some key concepts is in the set of slides

There is an unpublished manuscript of Chris Reedy from around 1974 that’s been circulating as an increasingly faded photocopy. It’s been typed into LaTeX, and the author has given permission for it to be posted on the net:

See

for errata and more.

Last revised on March 9, 2021 at 20:21:48. See the history of this page for a list of all contributions to it.