# nLab groupoid object in an (infinity,1)-category

group theory

## Internal $n$-category

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

(∞,1)-category theory

# Contents

## Idea

A group object in an ordinary category $C$ with pullbacks is an internal group. More generally, there is the notion of an internal groupoid in a category $C$.

By the logic of vertical categorification, an internal $\infty$-group or internal ∞-groupoid may be defined as a group(oid) object internal to an (∞,1)-category $C$ with (∞,1)-pullbacks. As described there, in full generality this involves not only a weakening of the usual associativity and unit laws up to homotopy, but requires specification of coherence laws of these homotopies up to higher homotopy, and so on.

A group object in an (∞,1)-category generalizes and unifies two familiar concepts:

Of particular relevance are such group objects that define effective quotients

A groupoid object is then accordingly the many-object version of a group object.

But notice the following. Since this is defined internal to an (∞,1)-category, externally these look like genuine ∞-groupoid and ∞-group objects. For instance a group object in a (2,1)-category such as Grpd is, externally, a 2-group.

Also notice that if the ambient $(\infty,1)$-category is in fact an (∞,1)-topos, then every object in there may already be thought of as an “∞-groupoid with geometric structure” (see for instance the discussion at cohesive (∞,1)-topos, but this is true more generally). The relation between the internal groupoid objects then and the objects themselves is (an oid-ification) of that of looping and delooping. Notably for $G$ any internal group object (externally an ∞-group) the corresponging ordinary object is its delooping object $\mathbf{B}G$, and every pointed connected object in the $(\infty,1)$-topos arises this way from an internal group object.

A groupoid object

$\cdots C_2 \stackrel{\to}{\stackrel{\to}{\to}} C_1 \stackrel{\to}{\to} C_0$

being effective means that it is the Čech nerve

$\cdots C_0 \times_{C_0//C_1} C_0 \times_{C_0//C_1} C_0 \stackrel{\to}{\stackrel{\to}{\to}} C_0 \times_{C_0//C_1} C_0 \stackrel{\to}{\to} C_0$

of its action groupoid $C_0//C_1$ (the (∞,1)-colimit over its diagram)

$\cdots C_2 \stackrel{\to}{\stackrel{\to}{\to}} C_1 \stackrel{\to}{\to} C_0 \to C_0//C_1 := colim_i C_i \,.$

Accordingly, groupoid objects in an $(\infty,1)$-category play a central role in the theory of principal ∞-bundles.

Notice that one of the four characterizing properties of an (∞,1)-topos by the higher analog of the Giraud theorem is that every groupoid object is effective.

## Definition (complete Segal-space style)

### Groupoid object

The following definition follows in style the definition of a complete Segal space object.

###### Definition

For $C$ an (∞,1)-category, a groupoid object in $C$ is a simplicial object in an (∞,1)-category

$A : \Delta^{op} \to C$

such that for all partitions $S \cup S'$ of $[n]$ that share precisely one vertex $s$, we have that

$\array{ A([n]) &\to & A(S) \\ \downarrow && \downarrow \\ A(S') &\to& A(\{s\}) }$

is a (∞,1)-pullback diagram in $C$. Here, by a partition $S \cup S'$ of $[n]$ that share precisely one vertex $s$, we mean two subsets $S$ and $S'$ of $\{0,1,\ldots,n\}$ whose union is $\{0,1,\ldots,n\}$ and whose intersection is the singleton $\{s\}$. The linear order on $[n]$ then restricts to the linear order on $S$ and $S'$.

The $(\infty,1)$-category of groupoid objects in $C$ is the full sub-(∞,1)-category

$Grpd(C) \hookrightarrow Func(\Delta^{op}, C)$

of the (∞,1)-category of (∞,1)-functors on those objects that are groupoid objects.

This is HTT, prop. 6.1.2.6, item 4'' with HTT, def. 6.1.2.7.

###### Remark

If one requires the above condition only for those partitions that are order-preserving, then this yields the definition of a (pre-)category object in an (∞,1)-category.

###### Definition

A groupoid object $A : \Delta^{op} \to C$ is the Cech nerve of a morphism $A_0 \to B$ if $A$ is the restriction of an augmented simplicial object $A^+ : \Delta^{op}_a \to C$ with $A^+_0 \to A^+_{-1}$ as the morphism $A_0 \to B$, such that the sub-diagram

$\array{ A^+_1 &\to& A^+_0 \\ \downarrow && \downarrow \\ A^+_0 &\to& A^+_{-1} }$

of $A^+$ is a (∞,1)-pullback diagram in $C$.

This is HTT, below prop. 6.1.2.11.

If $A$ is the Čech nerve of a morphism $A_0 \to A_{-1}$
then the groupoid object is deloopable in the groupoid sense.

###### Definition

A groupoid object $A : \Delta^{op} \to C$ is an effective quotient object if the (∞,1)-colimit diagram $A^+ : \Delta_a^{op} \to C$ exists, such that $A$ is the Cech nerve of $A^+_0 \to A^+_{-1}$, i.e. of $A_0 \to \lim_\to A_\bullet$.

### Group object

A group object is a groupoid object $U : \Delta^{op} \to C$ for which $U_0 \simeq *$ is a terminal object.

It follows (HTT, prop. 7.2.2.4) that a group object is of the form

$U = \left( \cdots G \times G \stackrel{\to}{\stackrel{\to}{\to}} G \stackrel{\to}{\to} * \right) \,.$

### Relation to $(\infty,1)$-quotients

###### Remark

A groupoid object in an $(\infty,1)$-category

$\left( \cdots A_2 \stackrel{\to}{\stackrel{\to}{\to}} A_1 \stackrel{\to}{\to} A_0 \right)$

is the (∞,1)-category analog of an internal equivalence relation on $A_0$, which is just a pair of morphisms

$R \stackrel{\to}{\to} A_0 \,.$

The colimit (coequalizer) of the latter diagram is the quotient of $A_0$ by the relation $R$.

Analogously, the (∞,1)-colimit

$\lim_\to (\Delta^{op} \stackrel{A}{\to} C)$

over the simplicial diagram $A : \Delta^{op} \to C$ is the corresponding $(\infty,1)$-quotient.

If we are given a model category presentation of the (∞,1)-category $C$, then this (∞,1)-colimit is presented by a homotopy colimit over the corresponding simplicial diagraM a homotopy quotient .

## Properties

### Equivalent characterizations

We state in prop. 1 below a list of equivalent conditions that characterize a simplicial object in an (∞,1)-category as a groupoid object. This uses the following basic notions, which we review here for convenience.

###### Definition

For $K \in$ sSet a simplicial set, write $\Delta_{/K}$ for its category of simplices. For $X_\bullet \in \mathcal{C}^{\Delta^{op}}$ a simplicial object, write

$X[K] \colon \Delta^{op}_{/K} \to \Delta^{op} \stackrel{X}{\to} \mathcal{C}$

for the precomposition of $X_\bullet$ with the canonical projection. Moreover, write

$X(K) \coloneqq \underset{\leftarrow}{\lim} X[K]$

for the (∞,1)-limit over this composite (∞,1)-functor in $\mathcal{C}$ (if it exists). (Notice: square brackets for the composite functor, round brackets for its $(\infty,1)$-limit.)

###### Remark

For $X_\bullet \in \mathcal{C}^{\Delta^{op}}$ and $K \to K'$ the following are equivalent

1. the induced morphism of cone $(\infty,1)$-categoris $\mathcal{C}_{X[K]} \to \mathcal{C}_{X[K']}$ is an equivalence of (∞,1)-categories;

2. the induced morphism of (∞,1)-limits $X(K) \to X(K')$ is an equivalence.

(The first perspective is used in (Lurie), the second in (Lurie2).)

###### Proof

In one direction: the limit is the terminal object in the cone category, and so is preserved by equivalences of cone categories. (This direction appears as (Lurie, prop. 4.1.1.8)). Conversely, the limits is the object representing cones, and hence an equivalence of limits induces an equivalence of cone categories.

###### Proposition

Let $\mathcal{C}$ be an $(\infty,1)$-category incarnated explicitly as a quasi-category. Then a simplicial object in $\mathcal{C}$ is a groupoid object if the following equivalent conditions hold.

1. If $K \to K'$ is a morphism in sSet which is a weak homotopy equivalence and a bijection on vertices, then the induced morphism on slice-(∞,1)-categories

$\mathcal{C}_{/X[K]} \to \mathcal{C}_{/X[K']}$
2. For every $n \geq 2$ and every $0 \leq i \leq n$, the morphism $\mathcal{C}_{/X[\Delta^n]} \to \mathcal{C}_{/X[\Lambda^n_i]}$ is an weak equivalence in the model structure for quasi-categories

3. (…)

Using remark 3 this means equivalently that the simplicial object $X_\bullet$ is a groupoid precisely if the following

1. $X_\bullet$ satisfies the ordinary Segal conditions and the morphism $X(\Delta^2) \to X(\Lambda^2_0)$ is an equivalence.

2. (…)

The first items appear as (Lurie, prop. 6.1.2.6). The second ones appear in the proofs of (Lurie2, prop. 1.1.8, lemma 1.2.25).

### The $(\infty,1)$-category of groupoid objects

###### Proposition

The $(\infty,1)$-category of groupoid objects in $C$ is a reflective sub-(∞,1)-category

$Grpd(C) \stackrel{\overset{}{\leftarrow}}{\hookrightarrow} Func(\Delta^{op}, C) \,.$

This is HTT, prop. 6.1.2.9. In nice cases the image of this reflective subcategory are the effective epimorphisms:

###### Proposition

If $C = \mathbf{H}$ in an (∞,1)-semitopos there is a natural equivalence of (∞,1)-categories

$Grpd(\mathbf{H}) \simeq (\mathbf{H}^I)_{eff}$

between the $(\infty,1)$-category of groupoid objects in $\mathbf{H}$ and the full sub-(∞,1)-category of the arrow category of $\mathbf{H}$ (the (∞,1)-functor (∞,1)-category $Func(\Delta[1], \mathbf{H})$) on the effective epimorphisms.

This appears below HTT, cor. 6.2.3.5.

### Cech nerves

Write $\Delta_a$ for the augmented simplex category (including the object $[-1]$).

###### Proposition

An augmented simplicial object $A^+ : \Delta_a^{op} \to C$ is the right Kan extension of its restriction to $[-1]$ and $[0]$

$\array{ \{[-1] \leftarrow [0]\} &\stackrel{A^+|_{\leq 0}}{\to}& C \\ \downarrow & \nearrow_{\mathrlap{A^+}} \\ \Delta_a^{op} }$

precisley if $A^+|_{\geq 0}$ is a groupoid object in $C$ and the diagram

$\array{ A_1 &\to& A_0 \\ \downarrow && \downarrow \\ A_0 &\to& A_{-1} }$

is a (∞,1)-pullback in $C$.

$A$ is called the Cech nerve of $A_0 \to A_{-1}$ if the equivalent conditions of this proposition are satisfied.

### Effective quotients

###### Proposition

In $C =$ ∞Grpd every groupoid object is an effective quotient, def. 3.

This is HTT, below remark 6.1.2.15 and HTT, cor. 6.1.3.20.

More generally, this is true for every (∞,1)-topos.

###### Proposition

In $C$ is an (∞,1)-topos, then every groupoid object in $C$ is an effective quotient, def. 3.

This is HTT, theorem 6.1.0.6 (4) iv).

### Delooping

For $\mathcal{X}$ an (∞,1)-category with (∞,1)-pullbacks and for $x : * \to X$ a pointed object in $\mathcal{X}$, its loop space object at $x$ is the (∞,1)-pullback

$\Omega_x X := {*} \prod_{X} {*}$

hence the object universally filling the diagram

$\array{ \Omega_x X &\to& * \\ \downarrow &\swArrow_{\simeq}& \downarrow^{\mathrlap{x}} \\ * &\stackrel{x}{\to}& X } \,.$

Since this is the beginning of the Cech nerve of $* \to X$, $\Omega_x X$ is naturally equipped with the structure of an $\infty$-group object in $\mathcal{X}$.

###### Proposition

Let $\mathcal{X}$ be an (∞,1)-topos. Then the operation of forming loop space objects constitutes an equivalence of (∞,1)-categories

$\Omega : PointedConnected(\mathcal{X}) \stackrel{\simeq}{\to} Grp(\mathcal{X})$

from the full sub-(∞,1)-category of the under-(∞,1)-category $*/\mathcal{X}$ of pointed objects on those that are also 0-connected (hence those that have an essentially unique point) with the $(\infty,1)$-category of group objects in $\mathcal{X}$.

This is HTT, lemma 7.2.2.11 (1)

The inverse to $\Omega$ we write

$\mathbf{B} : Grp(\mathcal{X}) \to PointedConnected(\mathcal{X}) \,.$

For $G \in Grp(\mathcal{X})$ we call $\mathbf{B}G$ its delooping.

## Examples

### Group objects in an $(\infty,1)$-topos

When the ambient (∞,1)-category is an (∞,1)-topos then – by the $\infty$-Giraud axioms – all groupoid objects are effective, meaning that for

$\mathbf{B}G = \lim_{\to} U_\bullet$

the (∞,1)-colimit over the group object $U_\bullet$ we have that $U_\bullet$ is reproduced as the Cech nerve of $* \to \mathbf{B}G$

$\left( \cdots G \times G \stackrel{\to}{\stackrel{\to}{\to}} G \stackrel{\to}{\to} * \right) \simeq \left( \cdots {*}\times_{\mathbf{B}G}{*}\times_{\mathbf{B}G}{*} \stackrel{\to}{\stackrel{\to}{\to}} {*}\times_{\mathbf{B}G}{*} \stackrel{\to}{\to} * \right) \,.$

The object $\mathbf{B}G$ is the delooping object of the group object $G$.

### Models for group objects in $\infty Grpd$

There is a model category structure that presents the (∞,1)-category of group objects in ∞Grpd: the ∞-groups.

• The group objects $G$ themselves are modeled by a model structure on the category $sGrp$ of simplicial groups.

• Their delooping spaces $\mathbf{B}G$ are modeled by a model structure on the category $sSet_0$ of simplicial sets with a single vertex.

The operation of forming loop space objects constitutes a Quillen equivalence between these two model structures

$\Omega : sSet_0 \stackrel{\simeq_{Quillen}}{\to} sGrp \,.$

The Quillen equivalence itself is in section 6 there.

###### Proposition

There exists the transferred model structure on the category $sGrp$ of simplicial groups along the forgetful functor

$U : sGrp \to sSet_{Quillen}$

to the standard model structure on simplicial sets.

This means that a morphism in $sGrp$ is a

• weak equivalences

• or fibration

precisely if it is so in $sSet_{Quillen}$.

This appears as (GoerssJardine, ch V, theorem. 2.3).

###### Proposition

There is a model structure on reduced simplicial sets $sSet_0$ (simplicial sets with a single vertex) whose

• weak equivalences

• and cofibrations

are those in the standard model structure on simplicial sets.

This appears as (GoerssJardine, ch V, prop. 6.2).

###### Proposition

The simplicial loop space functor $G$ and the delooping functor $\bar W(-)$ (discussed at simplicial group) constitute a Quillen equivalence

$(G \dashv \bar W) : sGr \stackrel{\overset{G}{\leftarrow}}{\underset{\bar W}{\to}} sSet_0 \,.$

The $(G \dashv \bar W)$-unit and counit are weak equivalences:

$X \stackrel{\simeq}{\to} \bar W G X$
$G \bar W G \stackrel{\simeq}{\to} G \,.$

This appears as (GoerssJardine, ch. V prop. 6.3).

## References

Groupoid objects in $(\infty,1)$-categories are the topic of section 6.1.2 in

Model category presentations of groupoid objects in $\infty Grpd$ by groupoidal complete Segal spaces are discussed in

• Adding inverses to diagrams encoding algebraic structures, Homology, Homotopy and Applications 10 (2008), no. 2, 149–174. (arXiv:0610291)

Adding inverses to diagrams II: Invertible homotopy theories are spaces, Homology, Homotopy and Applications, Vol. 10 (2008), No. 2, pp.175-193. (web, arXiv:0710.2254)

A standard textbook reference on the model categories presentation of $\infty$-groups in $sSet$ is chapter V of

Discussion from the point of view of category objects in an (∞,1)-category is in

Revised on October 22, 2013 20:21:40 by Colin Tan? (137.132.3.9)