nLab Segal category



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

Enriched category theory



The notion of Segal category is one of the models for that of (∞,1)-category, given by regarding an (,1)(\infty,1)-category as an ∞Grpd-enriched (∞,1)-category.

So the notion can be understood as modelling the notion of an sSet-enrichment up to coherent homotopy, i.e. a weak enrichment. As such it is closely related to the notion of complete Segal space, which models the notion of an internal category in sSet.

Indeed, Segal categories may be considered with enrichment not just over sSet, but over other suitable model categories. In particular, an iterated enrichment over itself gives rise to the notion of Segal n-category which is a model for (∞,n)-categories.

Since the major difference between (small) 𝒱\mathcal{V}-enriched categories and 𝒱\mathcal{V}-internal categories is that in the first case the objects (as opposed to all the hom objects) form an ordinary set, while in the second these form an object of 𝒱\mathcal{V}, too, accordingly the definition of Segal category is like that of (complete) Segal space, only that the simplicial set of objects is required to be an ordinary set (a discrete simplicial set).



A Segal category is

  • a simplicial simplicial set (\simeq bisimplicial set) X[Δ op,sSet]X \in [\Delta^{op}, sSet],

    where we call X 0X_0 the simplicial set of objects, X 1X_1 the simplicial set of morphisms; and X kX_k for k2k \geq 2 the simplicial set of sequences of composable morphisms of length kk;

  • such that X 0X_0 is a discrete (= constant) simplicial set;

  • and such that the Segal maps

    X Sp[k]Δ[k]:X kX 1× X 0× X 0X 1(kfactors) X^{Sp[k] \hookrightarrow \Delta[k]} : X_k \stackrel{\simeq}{\to} X_1 \times_{X_0} \cdots \times_{X_0} X_1 \;\;(k factors)

(With the defintion of X_k, shouldn’t X_k be equal to X_1 \times_{X_0}… X_1?)

induced by the spine inclusions Sp[k]Δ[k]Sp[k] \hookrightarrow \Delta[k] are weak equivalences of simplicial sets for k2k \geq 2.


There is no condition that a Segal category be fibrant with respect to the Reedy model structure on bisimplicial sets.


For XX a Segal category, the fiber product simplicial set X 1× X 0X 1X_1 \times_{X_0} X_1 is manifestly the space of pairs of composable 1-morphisms in XX, and the weak equivalence

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

given by the above definition together with the remaining face map d 1:X 2X 1d_1 : X_2 \to X_1 constitutes an ∞-anafunctor

:X 1× X 0X 1X 1 \circ : X_1 \times_{X_0} X_1 ⇸ X_1

given by the span

X 2 d 1 X 1 X 1× X 0X 1. \array{ X_2 &\stackrel{d_1}{\to}& X_1 \\ {}^{\mathllap{\simeq}}\downarrow \\ X_1 \times_{X_0} X_1 } \,.

This encodes the composition operation in the Segal category XX.

Accordingly, the analogous spans out of X kX_k for k3k \geq 3 encode the associativity of this composition as well as all its coherences.


Model category structure

The category of bisimplicial sets carries a model category structure whose fibrant objects are the Reedy fibrant Segal categories. This model structure for Segal categories is a presentation of the (∞,1)-category of (∞,1)-categories.

Operadic version

The operadic generalization of Segal category is that of Segal operad. Segal categories are precisely those Segal operads whose only inhabited operations-spaces are those of unary operations.


Inclusion of ordinary categories

Let CC be an ordinary small category and write N(C)sSetN(C) \in sSet for its nerve. Regard this as a bisimplicial set under the inclusion sSet[Δ op,Set][Δ op,sSet]sSet \simeq [\Delta^{op}, Set] \hookrightarrow [\Delta^{op}, sSet].

Then N(C)N(C) is a Segal category. Each simplicial set N(C) kN(C)_k is discrete, for all kk \in \mathbb{N}, and all the morphisms

N(C) kMor(C)× Obj(C)× Obj(C)Mor(C) N(C)_k \to Mor(C) \times_{Obj(C)} \cdots \times_{Obj(C)} Mor(C)

are in fact isomorphisms / bijections of sets. This property of the nerve of an ordinary category goes by the name Segal condition and is what gave Segal categories its name.

One may also form the nn-fold comma object-fiber product of a choice of base points π 0(C)C\pi_0(C) \to C with itself. This yields a Segal category incarnation of CC where in degree 1 we have the groupoid core of the arrow category of CC. For more on this see at Segal space – Examples - From a category.


The idea of Segal categories goes back (implicitly) to

and is explicit in

as kind of bisimplicial sets “which are ‘special’ in the sense that they satisfy a slight variation on a condition of Segal” (But the terminology “Segal category” is not used in Dwyer, Kan & Smith 1989.)

Discussion in the broader context of Segal n n -categories:

Discussion with emphasis on the comparison of the various model category presentations of ( , 1 ) (\infty,1) -categories:

Discussion in the context of enriched (∞,1)-categories:

Last revised on August 12, 2023 at 09:02:33. See the history of this page for a list of all contributions to it.