nLab complete Segal space



Internal (,1)(\infty,1)-Categories

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



A complete Segal space is a model for an internal category in an (∞,1)-category in ∞Grpd, with the latter presented by sSet/Top. So complete Segal spaces present (∞,1)-categories. They are also called Rezk categories after Charles Rezk.

More in detail, a complete Segal space XX is

such that

  1. there is a composition operation well defined up to coherent homotopy: exibited by the Segal maps

    X kX 1× X 0× X 0X 1 X_k \to X_1 \times_{X_0} \cdots \times_{X_0} X_1

    (into the iterated homotopy pullback of the ∞-groupoid of 1-morphisms over the \infty-groupoid of objects) being homotopy equivalences

    (so far this defines a Segal space);

  2. the notion of equivalence in X X_\bullet is compatible with that in the ambient ∞Grpd (“completeness”): the sub-simplicial object Core(X )Core(X_\bullet) on the invertible morphisms in each degree is homotopy constant: it has all face and degeneracy maps being homotopy equivalences.

    (this says that if a morphism is an equivalence under the explicit composition operation then it is already a morphism in X 0X_0 ).


We first discuss

as such, and then the more general notion of

internal to a suitable model category/(,1)(\infty,1)-category 𝒞\mathcal{C} – this reduces to the previous notion for 𝒞=sSet Quillen\mathcal{C} = sSet_{Quillen}.

Complete Segal spaces


A Segal space is a simplicial object in simplicial sets

X[Δ op,sSet] X \in [\Delta^{op}, sSet]

such that

  • it is fibrant in the Reedy model structure [Δ op,sSet Quillen] Reedy[\Delta^{op}, sSet_{Quillen}]_{Reedy};

  • it is a local object with respect to the spine inclusions {Sp[n]Δ[n]} n\{Sp[n] \hookrightarrow \Delta[n]\}_{n \in \mathbb{N}};

    equivalently: for all nn \in \mathbb{N} the Segal map

    X nX 1× X 0× X 0X 1 X_n \to X_1 \times_{X_0} \cdots \times_{X_0} X_1

    is a weak homotopy equivalence (of Kan complexes, in fact).

(Rezk, 4.1).


For XX a Segal space, its homotopy category Ho(X)Ho(X) is the Ho(Top)-enriched category whose objects are the vertices of X 0X_0

Obj(X)=(X 0) 0 Obj(X) = (X_0)_0

and for x,yObj(X)x,y \in Obj(X) the hom object is the homotopy type of the homotopy fiber product

Ho(X)(x,y):=π 0({x}× X 0X 1× X 0{y}). Ho(X)(x,y) := \pi_0 \Big(\{x\} \times_{X_0} X_1 \times_{X_0} \{y\}\Big) \,.

The composition

Ho X(x,y)×Ho X(y,z)Ho X(x,z) Ho_X(x,y) \times Ho_X(y,z) \to Ho_X(x,z)

is the (uniquely defined) action of the infinity-anafunctor

X 1× X 0X 1(d 2,d 0)X 2d 1X 1 X_1 \times_{X_0} X_1 \underoverset{\simeq}{(d_2, d_0)}{\leftarrow} X_2 \stackrel{d_1}{\to} X_1

on these connected components.

(Rezk, 5.3)


For XX a Segal space, write

X hoequX 1 X_{hoequ} \hookrightarrow X_1

for the inclusion of the connected components of those vertices that become isomorphisms in the homotopy category, def. .

(Rezk, 5.7)


A Segal space XX is called a complete Segal space if

s 0:X 0X hoequ s_0 : X_0 \to X_{hoequ}

is a weak equivalence.

(Rezk, 6.)


This condition is equivalent to XX being a local object with respect to the morphism N({01})*N(\{0 \stackrel{\simeq}{\to} 1\}) \to *. This is discussed below.


The completeness condition may also be thought of as univalence. See there for more.

Complete Segal space objects



Characterization of Completeness


A Segal space XX is a complete Segal space precisely if it is a local object with respect to the morphism N(01)*N(0 \stackrel{\simeq}{\to} 1) \to *, hence precisely if with respect to the canonical sSet-enriched hom objects we have that

X 0[Δ op,sSet](*,X)[Δ op,sSet](N(01),X) X_0 \simeq [\Delta^{op}, sSet](*, X) \to [\Delta^{op}, sSet](N(0 \stackrel{\simeq}{\to} 1), X)

is a weak equivalence.

(Rezk, theorem 6.2)

Model category structure

The category [Δ op,sSet][\Delta^{op}, sSet] of simplicial presheaves on the simplex category (bisimplicial sets) supports a model category structure whose fibrant objects are precisely the complete Segal spaces: the model structure for complete Segal spaces. This presents the (∞,1)-category of (∞,1)-categories.

Relation to simplicial localization

Given 𝒞\mathcal{C} a category with weak equivalences 𝒲Mor(𝒞)\mathcal{W} \subset Mor(\mathcal{C}) or more generally a “relative category”, then there is canonically a complete Segal space associated with it by the “relative nerve” construction, def. followed by fibrant replacement, and hence by the model structure this determines an (∞,1)-category N Rezk(𝒞,𝒲)N_{Rezk}(\mathcal{C},\mathcal{W}) .

On the other hand classical simplicial localization-theory provides several ways (e.g. hammock localization) to turn (𝒞,𝒲)(\mathcal{C}, \mathcal{W}) into an (∞,1)-category 𝒞[𝒲] 1\mathcal{C}[\mathcal{W}]^{-1} which universally turns the elements in 𝒲\mathcal{W} into homotopy equivalences.


These constructions are compatible in that there is an equivalence of (∞,1)-categories

N Rezk(𝒞,𝒲)𝒞[𝒲 1]. N_{Rezk}(\mathcal{C},\mathcal{W}) \simeq \mathcal{C}[\mathcal{W}^{-1}] \,.

For simplicial model categories this is (Rezk, theorem 8.3. For general model categories this is (Bergner 07, theorem 6.2). For the fully general case this follows from results by Clark Barwick, Daniel Kan and Bertrand Toën as pointed out by Chris Schommer-Pries here on MathOverflow.

Model categories for presheaves

There is a notion of right/left fibration of complete Segal spaces analogous to right/left Kan fibrations? for quasi-categories.


We discuss some examples. For more and more basic examples see also at Segal space – Examples.

Ordinary categories as complete Segal spaces

We discuss how an ordinary small category is naturally regarded as a complete Segal space. (Rezk, 3.5)


We need the following basic ingredients.

Write () ():Cat op×CatCat(-)^{(-)} : Cat^{op} \times Cat \to Cat for the internal hom in Cat, sending two categories AA, XX to the functor category X A=Func(A,X)X^A = Func(A,X).

By the discussion at nerve we have a canonical functor

ΔCat \Delta \hookrightarrow Cat

including the simplex category into Cat by regarding the simplex Δ[n]\Delta[n] as the category generated from nn consecutive morphisms.

The nerve itself is then then functor

N:CatsSet N : Cat \to sSet

to sSet sending a category CC to

N(C):kC Δ[k]. N(C) : k \mapsto C^{\Delta[k]} \,.

Its restriction along GrpdCatGrpd \hookrightarrow Cat to groupoids lands in Kan complexes KanCplxKanCplx \hookrightarrow sSet.

The core operation is the functor

Core:CatGrpd Core : Cat \to Grpd

right adjoint to the inclusion of Grpd into Cat. It sends a category to the groupoid obtained by discarding all non-invertible morphisms.

The construction

Let CC be a small category. Define

C[Δ op,sSet] \mathbf{C} \in [\Delta^{op}, sSet]


C k:=N(Core(C Δ[k])). \mathbf{C}_k := N(Core(C^{\Delta[k]})) \,.

In degree 0 this is the the core of CC itself. In degree 1 it is the groupoid C 1\mathbf{C}_1 underlying the arrow category of CC.

One sees that the source and target functors s,t:C Δ[1]Cs, t : C^{\Delta[1]} \to C are isofibrations and hence their image under core and nerve are Kan fibrations. Therefore it follows that the homotopy pullback (see there) C 1× C 0× C 0C 1\mathbf{C}_1 \times_{\mathbf{C}_0} \cdots \times_{\mathbf{C}_0} \mathbf{C}_1 is given already be the ordinary pullback in the 1-category Grpd. Using this, it is immediate that for all kk the functors

Core(C Δ[k])Core(C Δ[1])× Core(C)× Core(C)Core(C Δ[1]) Core(C^{\Delta[k]}) \to Core(C^{\Delta[1]}) \times_{Core(C)} \cdots \times_{Core(C)} Core(C^{\Delta[1]})

are isomorphisms, and so in particular

C kC 1× C 0× C 0C 1 \mathbf{C}_k \to \mathbf{C}_1 \times_{\mathbf{C}_0} \cdots \times_{\mathbf{C}_0} \mathbf{C}_1

is an equivalence.

It is clear that the composition operation in the complete Segal space defined this way “is” the composition in CC. In particular the morphisms that are invertible under this composition are precisely those that are already invertible in CC. Therefore we have the core simplicial object

Core(C):kN(Core(C) Δ[k])=N(Core(C)) Δ[k], Core(\mathbf{C}) : k \mapsto N(Core(C)^{\Delta[k]}) = N(Core(C))^{\Delta[k]} \,,

where, note, now we first take the core of CC and then form morphism categories.

This simplicial Kan complex has in each positive degree a path space object for the Kan complex N(Core(C))N(Core(C)).

Notably (since Δ[k]\Delta[k] is weak homotopy equivalent to the point) it follows that indeed all the face and degeneracy maps are weak homotopy equivalences.

So for every category CC, the simplicial object C\mathbf{C} constructed as above is a complete Segal space. This construction extends to a functor CatcompleteSegalSpaceCat \to completeSegalSpace and this is homotopy full and faithful.

Properties of the inclusion


Sing J:Cat[Δ op,sSet] Sing_J : Cat \to [\Delta^{op}, sSet]

for the functor just defined


For CC and DD two categories, there are natural isomorphisms

Sing J(C×D)Sing J(C)×Sing J(D) Sing_J(C \times D) \simeq Sing_J(C) \times Sing_J(D)


Sing J(D C)(Sing JD) Sing JC. Sing_J(D^C) \simeq (Sing_J D)^{Sing_J C} \,.

A functor f:CDf : C \to D is an equivalence of categories precisely if Sing J(f)Sing_J(f) is an equivalence in the Reedy model structure [Δ op,sSet] Reedy[\Delta^{op}, sSet]_{Reedy} (hence is degreewise a weak homotopy equivalence of Kan complexes).

This appears as (Rezk, theorem 3.7).

Relative and Model categories as complete Segal spaces

Let CC be a category with a class WMor(C)W \subset Mor(C) of weak equivalences. For instance, CC could be a model category or (much) more generally a “relative category”. Then the above construction has the following evident variant.


Let N(C,W)[Δ op,sSet]N(C,W) \in [\Delta^{op}, sSet] be given by

N(C,W):nN(Core W(C Δ[n])), N(C,W) : n \mapsto N(Core_W(C^{\Delta[n]})) \,,

where now Core W()Core_W(-) denotes the subcategory on those natural transformations whose components are weak equivalences in CC.


The typical model category is not a small category with respect to the base choice of universe. In this case N(C,W)N(C,W) will be a “large” bisimplicial set. In other words, one needs to employ some universe enlargement to interpret this definition.


If CC is a model category, then Core W(C Δ[n])Core_W(C^{\Delta[n]}) is the subcategory of weak equivalences in any of the standard model structures on functors on C Δ[n]C^{\Delta[n]}. By a classical fact discssed at (∞,1)-categorical hom-space, its nerve is a model for the core of the corresponding (∞,1)-category of (∞,1)-functors.

The bisimplicial set N(C,W)N(C,W) is not, in general, a complete Segal space. It does, however, represent the same (∞,1)-category as the simplicial localization of CC at WW; see this MO question.

We can, of course, always reflect N(C,W)N(C,W) into a complete Segal space by passing to a fibrant replacement in the model structure for complete Segal spaces. But something better is true here: it suffices to make a Reedy fibrant replacement (which does not change the homotopy type of the simplicial sets N(Core W(C Δ[n]))N(Core_W(C^{\Delta[n]})), but only “arranges them more nicely”).


Any Reedy fibrant replacement of N(C,W)N(C,W) is a complete Segal space.

This is (Rezk, theorem 8.3).

Quasi-categories as complete Segal spaces

The formula 𝒞kCore(𝒞 Δ[k])\mathcal{C} \mapsto k \mapsto Core(\mathcal{C}^{\Delta[k]}) also defines a relative functor from quasi-categories to complete segal spaces, which has a one-sided inverse XX ,0X \mapsto X_{\bullet,0}. This is the Γ\Gamma appearing in proposition 4.10 of the Joyal-Tierney reference.

However, to get a right Quillen functor, we need to use a different model of Core.



Δ J:ΔsSet \Delta_J : \Delta \to sSet

for the cosimplicial simplicial set that sends [n][n] to the nerve of the codiscrete groupoid on n+1n+1 objects

Δ J[n]=N(0n). \Delta_J[n] = N(0 \stackrel{\simeq}{\to} \cdots \stackrel{\simeq}{\to} n) \,.


Sing J:sSet[Δ op,sSet] Sing_J : sSet \to [\Delta^{op}, sSet]

for the functor given by

Sing J(X) n=Hom sSet(Δ[n]×Δ J[],X). Sing_J(X)_n = Hom_{sSet}(\Delta[n] \times \Delta_J[\bullet], X) \,.

For XsSetX \in sSet a quasi-category/inner Kan complex, Sing J(X)Sing_J(X) is a complete Segal space.

See at model structure for dendroidal complete Segal spaces the section Quasi-operads to dendroidal complete Segal spaces



Complete Segal spaces were originally defined in

  • Charles Rezk, A model for the homotopy theory of homotopy theory , Trans. Amer. Math. Soc., 353(3), 973-1007 (pdf)

The relation to quasi-categories is discussed in

Further discussion of the relation to simplicial localization is in

A survey of the definition and its relation to equivalent definitions is in section 4 of

See also pages 29 to 31 of

For literature on the variants and refinements see at Theta space and n-fold complete Segal space.

Related MathOverflow discussion includes

Groupoidal version

The groupoidal version of complete Segal spaces (that modelling just groupoid objects in an (∞,1)-category instead of general category objects in an (∞,1)-category) is discussed in

  • Julia Bergner, Adding inverses to diagrams encoding algebraic structures, Homology, Homotopy Appl. 10(2), 2008, 149-174 (arXiv:math/0610291)

  • Julia Bergner, Adding inverses to diagrams II: Invertible homotopy theories are spaces, Homology, Homotopy and Applications, vol. 10(1) 2008 (pdf)

Last revised on June 7, 2022 at 11:10:25. See the history of this page for a list of all contributions to it.