# nLab complete Segal space

Contents

## Internal $n$-category

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

(∞,1)-category theory

Background

Basic concepts

Universal constructions

Local presentation

Theorems

Extra stuff, structure, properties

Models

# Contents

## Idea

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 $X$ is

• for each $n \in \mathbb{N}$ a Kan complex $X_n$, thought of as the space of composable sequences of $n$-morphisms and their composites;

• forming a simplicial object $X_\bullet$ in sSet (a bisimplicial set);

such that

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

$X_k \to X_1 \times_{X_0} \cdots \times_{X_0} X_1$

(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 $X_\bullet$ is compatible with that in the ambient ∞Grpd (“completeness”): the sub-simplicial object $Core(X_\bullet)$ 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 $X_0$ ).

## Definition

We first discuss

as such, and then the more general notion of

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

### Complete Segal spaces

###### Definition
$X \in [\Delta^{op}, sSet]$

such that

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

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

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

$X_n \to X_1 \times_{X_0} \cdots \times_{X_0} X_1$

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

(Rezk, 4.1).

###### Definition

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

$Obj(X) = (X_0)_0$

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

$Ho(X)(x,y) := \pi_0 \Big(\{x\} \times_{X_0} X_1 \times_{X_0} \{y\}\Big) \,.$

The composition

$Ho_X(x,y) \times Ho_X(y,z) \to Ho_X(x,z)$

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

$X_1 \times_{X_0} X_1 \underoverset{\simeq}{(d_2, d_0)}{\leftarrow} X_2 \stackrel{d_1}{\to} X_1$

on these connected components.

###### Definition

For $X$ a Segal space, write

$X_{hoequ} \hookrightarrow X_1$

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

###### Definition

A Segal space $X$ is called a complete Segal space if

$s_0 : X_0 \to X_{hoequ}$

is a weak equivalence.

(Rezk, 6.)

###### Remark

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

###### Remark

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

(…)

## Properties

### Characterization of Completeness

###### Theorem

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

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

is a weak equivalence.

### Model category structure

The category $[\Delta^{op}, sSet]$ 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 $\mathcal{C}$ a category with weak equivalences $\mathcal{W} \subset Mor(\mathcal{C})$ 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 $N_{Rezk}(\mathcal{C},\mathcal{W})$ .

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

###### Theorem

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

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

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.

## Examples

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)

#### Preliminaries

We need the following basic ingredients.

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

By the discussion at nerve we have a canonical functor

$\Delta \hookrightarrow Cat$

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

The nerve itself is then then functor

$N : Cat \to sSet$

to sSet sending a category $C$ to

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

Its restriction along $Grpd \hookrightarrow Cat$ to groupoids lands in Kan complexes $KanCplx \hookrightarrow$ sSet.

The core operation is the functor

$Core : Cat \to Grpd$

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 $C$ be a small category. Define

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

by

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

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

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

$Core(C^{\Delta[k]}) \to Core(C^{\Delta}) \times_{Core(C)} \cdots \times_{Core(C)} Core(C^{\Delta})$

are isomorphisms, and so in particular

$\mathbf{C}_k \to \mathbf{C}_1 \times_{\mathbf{C}_0} \cdots \times_{\mathbf{C}_0} \mathbf{C}_1$

is an equivalence.

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

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

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

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

Notably (since $\Delta[k]$ 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 $C$, the simplicial object $\mathbf{C}$ constructed as above is a complete Segal space. This construction extends to a functor $Cat \to completeSegalSpace$ and this is homotopy full and faithful.

#### Properties of the inclusion

Write

$Sing_J : Cat \to [\Delta^{op}, sSet]$

for the functor just defined

###### Proposition

For $C$ and $D$ two categories, there are natural isomorphisms

$Sing_J(C \times D) \simeq Sing_J(C) \times Sing_J(D)$

and

$Sing_J(D^C) \simeq (Sing_J D)^{Sing_J C} \,.$

A functor $f : C \to D$ is an equivalence of categories precisely if $Sing_J(f)$ is an equivalence in the Reedy model structure $[\Delta^{op}, sSet]_{Reedy}$ (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 $C$ be a category with a class $W \subset Mor(C)$ of weak equivalences. For instance, $C$ could be a model category or (much) more generally a “relative category”. Then the above construction has the following evident variant.

###### Definition

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

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

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

###### Remark

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

###### Remark

If $C$ is a model category, then $Core_W(C^{\Delta[n]})$ is the subcategory of weak equivalences in any of the standard model structures on functors on $C^{\Delta[n]}$. 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 $N(C,W)$ is not, in general, a complete Segal space. It does, however, represent the same (∞,1)-category as the simplicial localization of $C$ at $W$; see this MO question.

We can, of course, always reflect $N(C,W)$ 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 $N(Core_W(C^{\Delta[n]}))$, but only “arranges them more nicely”).

###### Proposition

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

This is (Rezk, theorem 8.3).

### Quasi-categories as complete Segal spaces

The formula $\mathcal{C} \mapsto k \mapsto Core(\mathcal{C}^{\Delta[k]})$ also defines a relative functor from quasi-categories to complete segal spaces, which has a one-sided inverse $X \mapsto X_{\bullet,0}$. This is the $\Gamma$ appearing in Joyal & Tierney (2007), proposition 4.10.

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

###### Definition

Write

$\Delta_J : \Delta \to sSet$

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

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

Write

$Sing_J : sSet \to [\Delta^{op}, sSet]$

for the functor given by

$Sing_J(X)_n = Hom_{sSet}(\Delta[n] \times \Delta_J[\bullet], X) \,.$
###### Proposition

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

### General

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

• Julia Bergner, A survey of $(\infty, 1)$-categories (arXiv).