# nLab model structure on operads

model category

## Model structures

for ∞-groupoids

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

#### Higher algebra

higher algebra

universal algebra

# Contents

## Idea

If $V$ is a symmetric monoidal category that is also a monoidal model category, then under suitable conditions there is also the structure of a model category on the category of symmetric $V$-operads.

This is important for the notion of homotopy algebra over an operad, such as A-∞ algebras and E-∞ algebras.

## Definition

Throughout, let $V$ be a symmetric closed monoidal model category with all small colimits and finite limits.

### Symmetric collections

We first consider the collections of operations underlying a symmetric operad (with no notion of composition of operations yet).

For $G$ a discrete group write $\mathbf{B}G$ for the delooping groupoid: the category with a single object and $G$ as its set of morphisms. Then for $V$ any other category, write $V^{\mathbf{B}G}$ for the functor category, consisting of functors $\mathbf{B}G \to V$. This is the category of actions of $G$ on objects in $V$ (the category of representations).

For $G$ a finite group also $V^{\mathbf{B}G}$ inherits the structure of a closed symmetric monoidal category with small colimits and finite limits. There is a forgetful functor/free functor adjunction

$V^{\mathbf{B}G} \stackrel{(-)[G]}{\underset{U}{\to}} V \,.$

Write $\Sigma_n$ for the symmetric group on $n \in \mathbb{N}$ elements. Take $\Sigma_0$ and $\Sigma_1$ both to be the trivial group.

###### Definition

The category of collections (of potential operations) in $V$ is the product

$Coll(V) := \prod_{n \geq 0} V^{\mathbf{B}\Sigma_n} \,.$

A collection $P$ is a tuple of objects

$P = (P(n))_{n \in \mathbb{N}}$

each equipped with an action by the respective $\Sigma_n$.

### Hopf interval object

###### Definition

An object $H \in V$ is a Hopf algebra object if it is equipped with the structure of a monoid, that of a comonoid such that product and coproduct preserve each other.

If $V$ is equipped with a compatible structure of a monoidal model category we say that a a Hopf algebra object is an Hopf interval object if it is equipped with morphisms

$I \coprod I \hookrightarrow H \stackrel{\simeq}{\to} I$

that factor the codiagonal on $I$ by a cofibration followed by a weak equivalence.

###### Examples

Such cocommutative coalgebra intervals exist in

In

there is a coalgebra interval.

###### Remark

Since the coalgebra interval in the category of chain complexes is not cocommutative, this case requires special discussion, as some of the statements below will not apply to it. For more on this case see model structure on dg-operads.

### Model category structure

Assume now that $V$ is moreover equipped with a compatible structure of a monoidal model category.

###### Lemma

If $V$ is a cofibrantly generated model category, then for each finite group $G$ the transferred model structure on $V^{\mathbf{B}G}$ along the forgetful functor

$U : V^{\mathbf{B}G} \to V$

exists.

It follows that in this case the category of collections $Coll(V)$ is a cofibrantly generated model category where a morphisms is a fibration or weak equivalence if it is so degreewise in $V$, respectively.

###### Lemma

A $V$-operad is called $\Sigma$-cofibrant if its underlying collection is cofibrant in the above model stucture

A $V$-operad $P$ is called reduced if $P(0)$ is the tensor unit, $P(0) = I$. A morphism of reduced operads is one that is the identity on the 0-component.

###### Theorem

If

Then there exists a cofibrantly generated model category structure on the category of reduced $V$-operads, in which

• a morphism $P \to Q$ is a weak equivalence (resp. fibration) precisely if for all $n \gt 0$ the morphisms $P(n) \to Q(n)$ are weak equivalences (resp. fibrations) in $V$.
###### Proof

This is BergerMoerdijk, theorem 3.1.

If $V$ is even a cartesian closed category, a stronger statement is possible:

###### Theorem

Let $V$ be a cartesian closed category, such that

• $V$ is cofibrantly generated and the terminal object is cofibrant;

• $V$ has a symmetric monoidal fibrant replacement functor.

Then there exists a cofibrantly generated model structure on the category of $V$-operads, in which a morphism $P \to Q$ is a weak equivalence (resp. fibration) precisely if for all $n \geq 0$ the morphisms $P(n) \to Q(n)$ are weak equivalences (resp. fibrations) in $V$.

## Examples

The conditions of the above theorems are satisfied for

In these contexts,

• the associative operad is admissible $\Sigma$-cofibrant

• the commutative operad is far from being $\Sigma$-cofibrant.

This means we have rectification theorems for A-∞ algebras but not for E-∞ algebras. See model structure on algebras over an operad for more.

## Properties

### Cofibrancy

###### Proposition

Every cofibrant operad is also $\Sigma$-cofibrant.

This is (BergerMoerdijk, prop. 4.3).

###### Remark

The relevance of this is in section Homotopy algebras: this property enters the proof of the statement that the model structure on algebras over an operad over a $\Sigma$-cofibrant resolution is already Quillen equivalent to that of a full cofibrant resolution.

Many resolutions of operads that appear in the literature are in fact just $\Sigma$-cofibrant.

### Resolutions

We now discuss the construction and properties of cofibrant resolutions of operads and their algebras.

(assumptions now as at model structure on algebras over an operad)

First we describe free operads, and then Boardman-Vogt resolutions of operads, obtained from the construction of the free ones by adding labels for lengths in an interval object

###### Remark

The category of $C$-coloured operads is itself the category of algebras over a non-symmetric operad. See coloured operad for more. Thus the above theorem provides conditions under which $C$-coloured operads carry a model structure in which fibrations and weak equivalences are those morphisms of operads $P \to Q$ that are degreewise fibrations and weak equivalences in $\mathcal{E}$.

###### Terminology

We shall from now on call an operad $P$ cofibrant if the morphism $I_C \to P$ from the initial $C$-coloured operad has the left lifting property against degreewise acyclic fibrations of operads (irrespective of whether the above conditions for the existence of the model structure hold).

###### Theorem

The forgetful functor from $C$-colored operads to pointed $C$-colored collections has a left adjoint

$(F^*_C \dashv U_C) : Oper_C(\mathcal{E}) \stackrel{\leftarrow}{\to} Coll_C^*(\mathcal{E}) \,.$

This is (BergerMoerdijk, theorem 3.2).

###### Theorem

For each well-pointed $\Sigma$-cofibrant $C$-coloured operad $P$, the $(F^*_C \dashv U_C)$-counit factors as a cofibration followed by a weak equivalence

$F_C^*(P) \hookrightarrow W(H,P) \stackrel{\simeq}{\to} P$

of $C$-coloured operads, naturally in $P$ and $H$.

If $P \to Q$ is a $\Sigma$-cofibration between well-pointed $\Sigma$-cofibrant $C$-coloured operads, then the induced map $W(H,P) \to W(H,Q)$ is a cofibration of cofibrant $C$-coloured operads.

This is (BergerMoerdijk, theorem 3.5).

Here $W(H,P)$ is also called the coloured Boardman-Vogt resolution of $P$.

An algebra over an operad over $W(H,P)$ is called a $P$-algebra up to homotopy.

### Homotopy algebras over an operad

We discuss model structures on algebras over resolutions of operads. A more detailed treatment is at model structure on algebras over an operad.

With $V$ as above, say

###### Definition

A $V$-operad $P$ is admissible if the category of $P$-algebras carries a transferred model structure from the free functor/forgetful functor adjunction

$F_P : V \stackrel{\leftarrow}{\to} Alg_P : U_P \,.$

Under mild assumptions on $V$, cofibrant operads are admissible.

###### Definition

For an arbirtrary $V$-operad $P$, the category of homotopy $P$-algebras is the category of $\hat P$-algebras for some cofibrant replacement $\hat P$ of $P$.

Indeed, this is well defined up to Quillen equivalence:

Moreover, for this it is sufficient that $\hat P$ be $\Sigma$-cofibrant .

###### Proposition

If $V$ is a left proper model category with cofibrant unit, then for $\hat P$ a $\Sigma$-cofibrant resolution of $P$ (not necessarily fully cofibrant!) the category of $\hat P$ algebras is Quillen equivalent to that of homotopy $P$-algebras.

For instance the associative operad is $\Sigma$-cofibrant, so that by the above every $A-\infty$-algebra may be rectified to an ordinary monoid.

See around BerMor03, remark 4.6.

For more see model structure on algebras over an operad.

### Relation to dendroidal sets

For enrichment in $\mathcal{E} =$ Top or sSet, the dendroidal homotopy coherent nerve induces a Quillen equivalence between the model structure on coloured topological operads/simplicial operads and the model structure on dendroidal sets. (See there for more details.)

## References

An influential article in which many of the homotopical and $(\infty,1)$-categorical aspects of operad theory originate is

An early notion of resolution of operads in chain complexes is given in section 3 of

Cofibrant Boardman-Vogt resolutions of operads are discussed in

• Rainer Vogt, Cofibrant operads and universal $E_\infty$-operads , Bielefeld SB 343 (1999), to appear in Topology Appl.

A systematic study of model category structures on monochromatic symmetric operads and their algebras is in

The generalization to a model structure on coloured symmetric operads (symmetric multicategories) is discussed in

and independently in

And the generalization to colored operads over more general suitable enriching categories is in

• Giovanni Caviglia, A Model Structure for Enriched Coloured Operads (arXiv:1401.6983)

(generalizing acorresponding model structure on enriched categories).

An explicit construction of cofibrant resolution in this model structure and its relation to the original constructon of the Boardman-Vogt resolution is in

The induced model structures and their properties on algebras over operads are discussed in

The model structure on dg-operads is discussed in

Revised on December 22, 2016 13:01:12 by Anonymous (130.225.178.9)