# nLab 2-groupoid

### Context

#### Higher category theory

higher category theory

# Contents

## Idea

A $2$-groupoid is

## Definition

Fix a meaning/model of ∞-groupoid, however weak or strict you wish. Then a $2$-groupoid is an $\infty$-groupoid such that all parallel pairs of $j$-morphisms are equivalent for $j \geq 3$. Thus, up to equivalence, there is no point in mentioning anything beyond $2$-morphisms, except whether two given parallel $2$-morphisms are equivalent. This definition may give a concept more general than your preferred definition of $2$-groupoid, but it will be equivalent; basically, you may have to rephrase equivalence of $2$-morphisms as equality.

## Specific models

There are various objects that model the abstract notion of $2$-groupoid.

### Bigroupoids

A bigroupoid is a bicategory in which all morphisms are equivalences.

Bigroupoids may equivalently be thought of in terms of their Duskin nerves. These are precisely the Kan complexes that are 2-hypergroupoids.

### 2-Hypergroupoids

A $2$-hypergroupoid is a model for a 2-groupoid. This is a simplicial set, whose vertices, edges, and 2-simplices we identify with the objects, morphisms and 2-morphisms of the form

$\array{ && y \\ & \nearrow &\Downarrow& \searrow \\ x &&\stackrel{}{\to}&& z }$

in the 2-groupoid, respectively.

Moreover, the 3-simplices in the simplicial set encode the composition operation: given three composable 2-simplex faces of a tetrahedron (a 3-horn)

$\array{ y &\to& &\to& z \\ \uparrow &\seArrow& &\nearrow& \downarrow \\ \uparrow &\nearrow& &\Downarrow& \downarrow \\ x &\to&&\to& w } \;\;\; \;\;\; \array{ y &\to& &\to& z \\ &\searrow& &\swArrow& \downarrow \\ && &\searrow& \downarrow \\ &&&& w }$

the unique composite of them is is a fourth face $\kappa$ and a 3-cell $comp$ filling the resulting hollow tetrahedron:

$\array{ y &\to& &\to& z \\ \uparrow &\seArrow& &\nearrow& \downarrow \\ \uparrow &\nearrow& &\Downarrow& \downarrow \\ x &\to&&\to& w } \;\;\; \stackrel{comp}{\to} \;\;\; \array{ y &\to& &\to& z \\ \uparrow &\searrow& &\swArrow& \downarrow \\ \uparrow &{}_\kappa\Downarrow& &\searrow& \downarrow \\ x &\to&&\to& w } \,.$

The 3-coskeletal-condition says that every boundary of a 4-simplex made up of five such tetrahedra has a unqiue filler. This is the associativity coherence law on the comoposition operation:

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This says that any of the possible ways to use several of the 3-simpleces to compose a bunch of compsable 2-morphisms are actually equal.

### Homotopy 2-types

More generally one may consider a Kan complex that are just homotopy equivalent to a $3$-coskeletal one as a $2$-groupoid – precisely: as representing the same homotopy type, namely a homotopy 2-type.

## Strict $2$-groupoids

The general notion of $2$-groupoid above is also called weak $2$-groupoid to distinguish from the special case of strict 2-groupoids. A strict $2$-groupoid is a strict 2-category in which all morphisms are strictly invertible. This is equivalently a certain type of Grpd-enriched category.

## Examples

homotopy leveln-truncationhomotopy theoryhigher category theoryhigher topos theoryhomotopy type theory
h-level 0(-2)-truncatedcontractible space(-2)-groupoidtrue/unit type/contractible type
h-level 1(-1)-truncated(-1)-groupoid/truth value(0,1)-sheafmere proposition, h-proposition
h-level 20-truncatedhomotopy 0-type0-groupoid/setsheafh-set
h-level 31-truncatedhomotopy 1-type1-groupoid/groupoid(2,1)-sheaf/stackh-groupoid
h-level 42-truncatedhomotopy 2-type2-groupoid(3,1)-sheafh-2-groupoid
h-level 53-truncatedhomotopy 3-type3-groupoid(4,1)-sheafh-3-groupoid
h-level $n+2$$n$-truncatedhomotopy n-typen-groupoid(n+1,1)-sheafh-$n$-groupoid
h-level $\infty$untruncatedhomotopy type∞-groupoid(∞,1)-sheaf/∞-stackh-$\infty$-groupoid

Revised on February 17, 2014 20:19:32 by Mark Gomer? (68.54.30.66)