nLab
(infinity,1)-operad

Idea

The notion of $(\mathrm{\infty }\mathrm{,1})$-operad is to that of (∞,1)-category as operad is to category.

So, roughly, an $(\mathrm{\infty }\mathrm{,1})$-operad is an algebraic structure that has for each given type of input and one type of output an ∞-groupoid of operations that take these inputs to that output.

Definitions

Two models for $(\mathrm{\infty }\mathrm{,1})$-operads exist to date, one by Cisinski–Moerdijk–Weiss, the other by Lurie.

The first one models $(\mathrm{\infty }\mathrm{,1})$-operads as dendroidal sets in close analogy to (in fact as a generalization of how) simplicial sets model (∞,1)-categories.

The second models the (∞,1)-category version of a category of operators of an operad.

In terms of dendroidal sets

Here simplicial sets are generalized to dendroidal sets. The theory of $(\mathrm{\infty }\mathrm{,1})$-operads is then formulated in terms of dendroidal sets in close analogy to how the theory of (∞,1)-categories is formulated in terms of simplicial sets.

There is a model structure on dendroidal sets whose fibrant objest are the quasi-operads in direct analogy to the notion of quasi-category.

Here are two blog entries on talks on this stuff:

• Dendroidal Sets and Infinity-Operads

• Moerdijk on Infinity Operads

In terms of $(\mathrm{\infty }\mathrm{,1})$-categories of operators

Every operad $A$ encodes and is encoded by its category of operators $C_A$. In the approach to $(\mathrm{\infty }\mathrm{,1})$-operators described below, the notion of category of operators is generalized to an (∞,1)-category of operators.

In this approach an $(\mathrm{\infty }\mathrm{,1})$-operad $C^{\otimes }$ is regarded as an (∞,1)-category $C$ – the unary part of the $(\mathrm{\infty }\mathrm{,1})$-operad to be described– with extra structure that determines (∞,1)-functors $C^{\times n}\to C$.

This and the conditions on these are encoded in requiring that $C^{\otimes }$ is an $(\mathrm{\infty }\mathrm{,1})$-functor $C^{\otimes }\to \Gamma$ over Segal's category? $\Gamma$ of pointed finite sets, satisfying some conditions.

In particular, any symmetric monoidal (∞,1)-category yields an example of an $(\mathrm{\infty }\mathrm{,1})$-operad in this sense. In fact, symmetric monoidal $(\mathrm{\infty }\mathrm{,1})$-categories can be defined as $(\mathrm{\infty }\mathrm{,1})$-operads such that the functor $C^{\otimes }\to \Gamma$ is a coCartesian fibration. (For the moment, see monoidal (infinity,1)-category for more comments and references on higher operads in this context.)

This is the approach described in (the new version of !)

• Jacob Lurie, Commutative algebra (pdf)

Model for $(\mathrm{\infty }\mathrm{,1})$-categories of operators

There is a model category that presents the (∞,1)-category $(\mathrm{\infty }\mathrm{,1}){\mathrm{Cat}}_{\mathrm{Oper}}$ of $(\mathrm{\infty }\mathrm{,1})$-categories of operations.

Proposition

There exists a

• left proper

• combinatorial

model category $𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$

• whose underlying category has

  • objects are marked simplicial set $S$ equipped with a morphism $S\to N({\mathrm{FinSet}}_{*})$ such that marked edges map to inert morphisms in ${\mathrm{FinSet}}_{*}$ (those for which the preimage of te marked point contains just the marked point)

  • morphisms are morphisms of marked simplicial sets $S\to T$ such that the triangle

    $$\begin{array}{ccccc}S& & \to & & T\\ & \searrow & & \swarrow \\ & & N({\mathrm{FinSet}}_{*})\end{array}$$\array{ S &&\to&& T \\ & \searrow && \swarrow \\ && N(FinSet_*) }

    commutes;

• which is canonically an SSet-enriched category;

• and whose model structure is given by

  • cofibrations are those morphisms whose underlying morphisms of simplicial sets ate cofibrations, hence monomorphisms

  • weak equivalences are those morphisms $S\to T$ such that for all $A\to N({\mathrm{FinSet}}_{*})$ that are $(\mathrm{\infty }\mathrm{,1})$-categories of operations by the above definition, the morphism of SSet-hom objects

    $$𝒫{\mathrm{Op}}_{\mathrm{\infty }}(T,A)\to 𝒫{\mathrm{Op}}_{\mathrm{\infty }}(S,A)$$\mathcal{P}Op_\infty(T,A) \to \mathcal{P}Op_\infty(S,A)

    is a homotopy equivalence of simplicial sets.

  • an object is fibrant if and only if it is an $(\mathrm{\infty }\mathrm{,1})$-category of operations, by the above definition.

Proof

This is prop 1.8 4 in

• Jacob Lurie, Commutative algebra (pdf)

Relation between the two definitions

At the time of this writing there is no discussion in “the literature” of the relation between the definition of $(\mathrm{\infty }\mathrm{,1})$-operads in terms of dendroidal sets (Cisinski, Moerdijk, Weiss) and $(\mathrm{\infty }\mathrm{,1})$-categories of operators (Lurie). The following are some tentative observations. - Urs

There is an obvious way to regard a tree as an $(\mathrm{\infty }\mathrm{,1})$-category of operators:

Definition

(dendroidal $(\mathrm{\infty }\mathrm{,1})$-category of operators)

Let

$$\omega :\Omega \hookrightarrow \mathrm{Op}\stackrel{C_{(-)}}{\to }\mathrm{Cat}/{\mathrm{FinSet}}_{*}\stackrel{N}{\to }𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$$\omega : \Omega \hookrightarrow Op \stackrel{C_{(-)}}{\to} Cat/FinSet_* \stackrel{N}{\to} \mathcal{P}Op_{(\infty,1)}

be the dendroidal object given by the following composition:

• $\Omega \hookrightarrow \mathrm{Op}$ is the functor from the tree category $\Omega$ to the category of symmetric colored operads (over Set) that sends a tree to the operad freely generated from it;

• $\mathrm{Op}\stackrel{C_{(-)}}{\to }\mathrm{Cat}/{\mathrm{FinSet}}_{*}$ sends an operad to its category of operators;

• $\mathrm{Cat}/{\mathrm{FinSet}}_{*}\stackrel{N}{\to }𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$ takes the nerve of this category, regarded as a marked simplicial set over $N({\mathrm{FinSet}}_{*})$, whose marked edges are the inert morphisms in the category of operations.

Following the general pattern of nerve and realization, we get:

Definition

(dendroidal nerve of Lurie-$\mathrm{\infty }$-operad)

The functor

$$N_d:={\mathrm{Hom}}_{𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}}(\omega (-),-):𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}\to \mathrm{dSet}$$N_d := Hom_{\mathcal{P}Op_{(\infty,1)}}(\omega(-), -): \mathcal{P}Op_{(\infty,1)} \to dSet

that sends a marked simplicial set $A\to N({\mathrm{FinSet}}_{*})$ to the dendroidal set which sends a tree $T$ to the set of morphisms of $\omega (T)$ into $A$

$$N_d(A):T\mapsto {\mathrm{Hom}}_{𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}}(\omega (T),A)$$N_d(A) : T \mapsto Hom_{\mathcal{P}Op_{(\infty,1)}}(\omega(T), A)

is the dendroidal nerve of $A$.

One expects that $N_d$ induces a Quillen adjunction and indeed a Quillen equivalence between the above model category structure on $𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$ and the model structure on dendroidal sets. The following is as far as I think I can prove aspects of this. -Urs.

Proposition

The dendroidal nerve functor has the following properties:

• it is the right adjoint of a SSet-enriched adjunction

  $$C_{(-)}:\mathrm{dSet}\overleftarrow{\to }𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}:N_d$$C_{(-)} : dSet \stackrel{\leftarrow}{\to} \mathcal{P}Op_{(\infty,1)} : N_d

• it sends fibrant objects to fibrant objects

  i.e. it sends $(\mathrm{\infty }\mathrm{,1})$-categories of operations to $(\mathrm{\infty }\mathrm{,1})$-operads in their incarnation as “quasi-operads”;

• it sends objects $\pi :A\to N({\mathrm{FinSet}}_{*})$ that come from grouplike symmetric monoidal ∞-groupoids to fully Kan dendroidal sets (that have the extension property with respect to all horns)

• it sends objects $\pi :A\to N({\mathrm{FinSet}}_{*})$ that come from symmetric monoidal (∞,1)-categories to dendroidal sets that have the extension property with respect to at least one outer horn ${\Lambda }_{v}T$ for $v\in T$ an $n$-corolla, for all $n\in \mathbb{N}$.

• its left adjoint sends cofibrations to cofibrations and acyclic cofibrations with cofibrant domain to acyclic cofibrations.

Proof

respect for fibrant objects. If $A\to N({\mathrm{FinSet}}_{*})$ is fibrant, then in particual $A$ is a weak Kan complex hence has the extension property with respect to all inner horn inclusions of simplices. We need to show that this implies that $N_d(A)$ has the extension property with respect to all inner horn inclusions of trees.

By an (at the moment unpublished) result by Moerdijk, right lifting property with respect to inner horn inclusions of trees is equivalend to right lifting property with respect to inclusions of spines of trees: the union over all the corollas in a tree.

For this the extension property means that if we find a collection $\{C_{k_i}\to N_d(A)\}=\mathrm{Sp}(T)$ of corollas in $N_d(A)$ that match at some inputs and output, then these can be composed to an image $T\to N_d(A)$ of the corresponding tree $T$ in $N_d(A)$.

An image of $T$ in $N_d(A)$ is an image of $\omega (T)$ in $A$. In the category of operators $\omega (A)$ every tree may be represented as the composite of a sequence of morphisms each of which consists of precisely one of the corollas $C_{k_i}$ in parallel to identity morphisms. This way gluing the tree from the corollas is a matter of composing a sequence of edges in $A$. But this is guaranteed to be possible if $A$ is a weak Kan complex.

symmetric monoidal product and outer horn lifting

As described at cartesian morphism, an edge $f:{\Delta }^1\to A$ in $A$ is coCartesian if for all diagrams

$$\begin{array}{c}{\Delta }^{\mathrm{0,1}}\\ \downarrow & {\searrow }^f\\ {\Lambda }_0^n& \to & A\\ \downarrow & & \downarrow \\ {\Delta }^n& \to & N({\mathrm{FinSet}}_{*})\end{array}$$\array{ \Delta^{0,1} \\ \downarrow & \searrow^f \\ \Lambda^n_0 &\to & A \\ \downarrow && \downarrow \\ \Delta^n &\to& N(FinSet_*) }

of 0-horn lifting problems where the first edge of the horn is $f$ itself, there exists a lift

$$\begin{array}{c}{\Delta }^{\mathrm{0,1}}\\ \downarrow & {\searrow }^f\\ {\Lambda }_0^n& \to & A\\ \downarrow & \nearrow & \downarrow \\ {\Delta }^n& \to & N({\mathrm{FinSet}}_{*})\end{array}.$$\array{ \Delta^{0,1} \\ \downarrow & \searrow^f \\ \Lambda^n_0 &\to & A \\ \downarrow &\nearrow & \downarrow \\ \Delta^n &\to& N(FinSet_*) } \,.

For $f$ the parallel application of an $n$-corolla with a collection of identity morphisms this implies that any outer horn ${\Lambda }_{v}T\to N_d(A)$ for which the vertex $v:C_n\to N_d(A)$ maps to $f$, the dendroidal set $N_d(A)$ has the extension property with respect to the inclusion ${\Lambda }_{d}T\hookrightarrow T$.

the left adjoint and its respect for cofibrations

By general nonsense the left adjoint to $N_d$ is given by the coend

$$C_{(-)}:\mathrm{dSet}\to 𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$$C_{(-)} : dSet \to \mathcal{P}Op_{(\infty,1)}

$$C_P={\int }^{T\in \Omega }\omega (T)\cdot P(T),$$C_P = \int^{T \in \Omega} \omega(T) \cdot P(T) \,,

where in the integrand we have the tautological tensoring of $𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$ over Set.

Notice that $\omega :\Omega \to 𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$ is an SSet-enriched functor for the ordinary category $\Omega$ regarded as a simplicially enriched category by the canonical embedding $\mathrm{Set}\hookrightarrow \mathrm{SSet}$. Therefore this adjunction $F⊣N_d$ is defined entirely in SSet-enriched category theory and hence is a simplicial adjunction.

The model structure on dendroidal sets has a set of generating cofibrations given by the boundary inclusions of trees. $\partial \Omega [T]\hookrightarrow \Omega [T]$. Tese evidenly map to monomorphisms of underlying simplicial sets under $F$, hence to cofibrations.

For $f:P\hookrightarrow Q$ an acyclic cofibration with cofibrant domain, we need to check that $C_f:C_X\to C_Y$ is a weak equivalence in $𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}$. This is by definition the case if for every fibrant object $A$ the morphism

$$𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}(C_Y,A)\to 𝒫{\mathrm{Op}}_{(\mathrm{\infty }\mathrm{,1})}(C_X,A)$$\mathcal{P}Op_{(\infty,1)}(C_Y,A) \to \mathcal{P}Op_{(\infty,1)}(C_X,A)

is a weak equivalence in the standard model structure on simplicial sets. By the simplicial adjunction $F⊣N_d$ this is equivalent to

$$\mathrm{dSet}(f,N_d(A)):\mathrm{dSet}(Y,N_d(A))\to \mathrm{dSet}(X,N_d(A))$$dSet(f,N_d(A)) : dSet(Y,N_d(A)) \to dSet(X,N_d(A))

being a weak equivalence. By the above $N_d(A)$ is fibrant. By section 8.4 of the lecture notes on dendroidal sets cited at model structure on dendroidal sets a morphism between cofibrant dendroidal sets is a weak equivalence precisely if homming it into any fibrant dendroidal set produces an equivalence of homotopy categories.

Since $f$ is a weak equivalence between cofibrant objects by assumption, it follows that indeed $\mathrm{dSet}(f,N_d(A))$ is a weak equivalence for all fibrant $A$.

(AHM, or does it? there is a prob here, but I need to run now…)

Hence $C_f$ is a weak equivalence.