model structure on operads


Model category theory

model category, model \infty -category



Universal constructions


Producing new model structures

Presentation of (,1)(\infty,1)-categories

Model structures

for \infty-groupoids

for ∞-groupoids

for equivariant \infty-groupoids

for rational \infty-groupoids

for rational equivariant \infty-groupoids

for nn-groupoids

for \infty-groups

for \infty-algebras



for stable/spectrum objects

for (,1)(\infty,1)-categories

for stable (,1)(\infty,1)-categories

for (,1)(\infty,1)-operads

for (n,r)(n,r)-categories

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

Higher algebra



If VV 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 VV-operads.

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


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

For GG a finite group also V BGV^{\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 BGU()[G]V. V^{\mathbf{B}G} \stackrel{(-)[G]}{\underset{U}{\to}} V \,.

Write Σ n\Sigma_n for the symmetric group on nn \in \mathbb{N} elements. Take Σ 0\Sigma_0 and Σ 1\Sigma_1 both to be the trivial group.


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

Coll(V):= n0V BΣ n. Coll(V) := \prod_{n \geq 0} V^{\mathbf{B}\Sigma_n} \,.

A collection PP is a tuple of objects

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

each equipped with an action by the respective Σ n\Sigma_n.

Hopf interval object


An object HVH \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 VV 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

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

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


Such cocommutative coalgebra intervals exist in


there is a coalgebra interval.


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 VV is moreover equipped with a compatible structure of a monoidal model category.


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

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


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


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

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



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

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

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


Let VV be a cartesian closed category, such that

Then there exists a cofibrantly generated model structure on the category of VV-operads, in which a morphism PQP \to Q is a weak equivalence (resp. fibration) precisely if for all n0n \geq 0 the morphisms P(n)Q(n)P(n) \to Q(n) are weak equivalences (resp. fibrations) in VV.


The conditions of the above theorems are satisfied for

In these contexts,

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




Every cofibrant operad is also Σ\Sigma-cofibrant.

This is (BergerMoerdijk, prop. 4.3).


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 Σ\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 Σ\Sigma-cofibrant.


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


The category of CC-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 CC-coloured operads carry a model structure in which fibrations and weak equivalences are those morphisms of operads PQP \to Q that are degreewise fibrations and weak equivalences in \mathcal{E}.


We shall from now on call an operad PP cofibrant if the morphism I CPI_C \to P from the initial CC-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).


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

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

This is (BergerMoerdijk, theorem 3.2).


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

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

of CC-coloured operads, naturally in PP and HH.

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

This is (BergerMoerdijk, theorem 3.5).

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

An algebra over an operad over W(H,P)W(H,P) is called a PP-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 VV as above, say


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

F P:VAlg P:U P. F_P : V \stackrel{\leftarrow}{\to} Alg_P : U_P \,.

Under mild assumptions on VV, cofibrant operads are admissible.


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

Indeed, this is well defined up to Quillen equivalence:

BerMor03, corollary 4.5.

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


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

For instance the associative operad is Σ\Sigma-cofibrant, so that by the above every AA-\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.)


An influential article in which many of the homotopical and (,1)(\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

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

(generalizing a corresponding 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