model structure for Segal operads


Model category theory

model category



Universal constructions


Producing new model structures

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

Model structures

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Higher algebra



The model structure for Segal operads is a presentation of the (∞,1)-category of (∞,1)-operads regarding these as ∞Grpd-enriched operads.

It is the operadic analog of the model structure for Segal categories: its fibrant objects are operadic analogs of Segal categories.


Write Ω\Omega for the tree category, the site for dendroidal sets.

Segal operads

Write η\eta for the tree with a single edge and no vertices. Write

sdSet:=[Ω op,sSet] sdSet := [\Omega^{op}, sSet]

for the category of simplicial presheaves on the tree category – simplicial dendroidal sets or dendroidal simplicial sets (see model structure for complete dendroidal Segal spaces for more on this).


A Segal pre-operad X[Ω op,sSet]X \in [\Omega^{op}, sSet] is a simplicial dendroidal set such that X(η)X(\eta) is a discrete simplicial set (a plain set regarded as a simplicially constant simplicial set). Write

SegalPreOperad[Ω op,sSet] SegalPreOperad \hookrightarrow [\Omega^{op}, sSet]

for the full subcategory on the Segal pre-operads.

A Segal operad is a Segal pre-operad such that for every tree TΩT \in \Omega the powering

X Ω[T]X Sp(T)sSet X^{\Omega[T]} \to X^{Sp(T)} \in sSet

of the spine inclusion (Sp(T)T)(Sp(T) \hookrightarrow T) \in dSet into XX is an acyclic Kan fibration. Write

SegalOperadSegalPreOperad SegalOperad \hookrightarrow SegalPreOperad

for the full subcategory on the Segal operads.

A Reedy-fibrant Segal operad is a Segal operad which is moreover fibrant in the generalized Reedy model structure [Ω op,sSet] gReedy[\Omega^{op}, sSet]_{gReedy}.

This is (Cisinski-Moerdijk, def. 7.1, def. 8.1).


The definition of Segal pre-operads encodes a set of colors of an operad, together with for each tree TT an ∞-groupoid of operations in the operad of the shape of this tree — notably \infty-groupoids of nn-ary operations if the tree is the nn-corolla, T=C nT = C_n.

The condition on Segal operads encodes the existence of composition of these operad operations by ∞-anafunctors. See the discussion at Segal category for more on this.

The Reedy fibrancy condition is mostly a technical convenience.


The inclusion def. 1 has a left and right adjoint functors

sdSetγ *γ *γ !SegalPreOperad. sdSet \stackrel{\overset{\gamma_!}{\to}}{\stackrel{\overset{\gamma^*}{\leftarrow}}{\underset{\gamma_*}{\to}}} SegalPreOperad \,.

One way to see the existence of the adjoints is to note that SegalPreOperadSegalPreOperad is a category of presheaves over the site S(Ω)S(\Omega) which is the localization of Ω×Δ\Omega \times \Delta at morphisms of the form (,Id η)(-,Id_\eta), where η\eta is the tree with one edge and no vertex. Write

γ:Δ×ΩS(Ω) \gamma : \Delta \times \Omega \to S(\Omega)

for the localization functor, then the inclusion of Segal pre-operads is the precomposition with this functor

γ *:SegalPreOperad[S(Ω) op,sSet][Ω op,sSet]. \gamma^* : SegalPreOperad \simeq [S(\Omega)^{op}, sSet] \hookrightarrow [\Omega^{op}, sSet] \,.

Therefore the left and right adjoint to γ *\gamma^* are given by left and right Kan extension along γ\gamma.

Explicitly, these adjoints are given as follows.

For X[Ω op,sSet]X \in [\Omega^{op}, sSet], the Segal pre-operad γ !(X)\gamma_!(X) sends a tree TT either to X(T)X(T), if TT is non-linear, hence if it admits no morphism to η\eta, or else to the pushout

X(η) X(T) π 0X(η) γ !(X)(T) \array{ X(\eta) &\to& X(T) \\ \downarrow && \downarrow \\ \pi_0 X(\eta) &\to& \gamma_!(X)(T) }

in sSet, where the top morphism is X(Tη)X(T \to \eta) for the unique morphism to η\eta.

In words, γ !(X)\gamma_!(X) is obtained from XX precisely by contracting the simplicial set of colors to its set of connected components.

Special morphisms

We discuss morphisms between Segal pre-operads with special properties, which will appear in the model structure.


Say a morphism ff in SegalPreOperadSegalPreOperad is a normal monomorphism precisely if γ *(f)\gamma^*(f) is a normal monomorphism (see generalized Reedy model structure), which in turn is the case if it is simplicial-degreewise a normal morphisms of dendroidal sets (see there for details).

Correspondingly, a Segal pro-operad XX is called normal if X\emptyset \to X is a normal monomorphism.


A morphism in SegalPreOperadSegalPreOperad is called an acyclic fibration precisely if it has the right lifting property against all normal monomorphisms, def. 2.


Say a morphism ff in SegalPreOperadSegalPreOperad is a Segal weak equivalence precisely if γ *(f)\gamma^*(f) is a weak equivalence in the model structure for dendroidal complete Segal spaces [Ω opm,sSet] gReedycSegal[\Omega^{op}m, sSet]_{gReedy \atop cSegal}.


Call a morphism in SegalPreOperadSegalPreOperad

  • a weak equivalence precisely if it is a Segal weak equivalence, def. 4;

  • a cofibration precisely if it is a normal monomorphism, def. 2.

Theorem 1 below asserts that this is indeed a model category struture whose fibrant objects are the Segal operads.


Of the various classes of morphisms


If f:XYf : X \to Y in [Ω op,sSet][\Omega^{op}, sSet] is a normal monomorphism and π 0X(η)π 0Y(η)\pi_0 X(\eta) \to \pi_0 Y(\eta) is a monomorphism, then γ !(f)\gamma_!(f) is normal in SegalPreOperadSegalPreOperad.

(Cis-Moer, lemma 7.4).


The class of normal monomorphisms in SegalPreOperadSegalPreOperad is generated (under pushout, transfinite composition and retracts) by the set

{γ !(Δ[n]×Ω[T]Δ[n]×Ω[T])γ !(Δ[n],Ω[T])} nΔ,TΩ,T1{η} \{ \gamma_!(\partial \Delta[n] \times \Omega[T] \cup \Delta[n] \times \partial \Omega[T]) \to \gamma_! (\Delta[n], \Omega[T]) \}_{n \in \Delta, T \in \Omega, {\vert T\vert} \geq 1} \cup \{ \emptyset \to \eta \}

(Cis-Moer, prop 7.5).


Let X[Ω op,sSet] gReedySegalX \in [\Omega^{op}, sSet]_{gReedy \atop Segal} be fibrant. Then γ *X\gamma_* X is a Reedy fibrant Segal operad. If XX is moreover fibrant in [Ω op,sSet] gReedycSegal[\Omega^{op}, sSet]_{gReedy \atop cSegal} then the counit γ *γ*XX\gamma^* \gamma* X \to X is a weak equivalence in [Ω op,sSet] gReedycSegal[\Omega^{op}, sSet]_{gReedy \atop cSegal}.

(Cis-Moer, prop 8.2).


An acyclic fibration in SegalPreOperadSegalPreOperad, def. 3, is also a weak equivalence in [Ω op,sSet] gReedySegal[\Omega^{op}, sSet]_{gReedy \atop Segal}.

(Cis-Moer, prop 8.12).

Of the model structure itself


The structures in def. 5 make the category SegalPreOperadSegalPreOperad a model category which is

This is (Cis-Moer, theorem 8.13).


The existence of the cofibrantly generated model structure follows with Smith’s theorem: by the discussion there it is sufficient to notice that

  1. the Segal equivalences are an accessibly embedded accessible full subcategory of the arrow category;

  2. the acyclic cofibrations are closed under pushout and retract;

    (both of these because these morphisms come from the combinatorial model category [Ω op,sSet] gReedycSegal[\Omega^{op}, sSet]_{gReedy \atop cSegal})

  3. the morphisms with right lifting against the normal monomorphisms are weak equivalences, by lemma 2.

Relation to other model structures

We discuss the relation to various other model structures for operads. For an overview see table - models for (infinity,1)-operads.

To dendroidal complete Segal spaces

(…) model structure for dendroidal complete Segal spaces


Revised on April 2, 2012 15:12:11 by Urs Schreiber (