# nLab center of an infinity-group

### Context

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

(∞,1)-category theory

# Contents

## Idea

The generalization of the notion of center from groups to ∞-groups.

## Definition

For $G \in \infty Grp(\mathcal{X})$ an ∞-group, there is a canonical morphism

$AUT(G) \to Out(G)$

from its automorphism ∞-group $AUT(G) := \underline{Aut}_{\mathcal{X}}(\mathbf{B}G)$ to its outer automorphism ∞-group.

The homotopy fiber of this morphism

$\mathbf{B} Z(G) \to Aut(G) \to Out(G)$

is the delooping of an ∞-group $Z(G)$. This is the center of $G$.

## Examples

### Centers of ordinary groups

For $\mathcal{X} =$ ∞Grpd and $G$ 0-truncated, it is an ordinary discrete group. Its automorphism 2-group is the strict 2-group coming from the crossed module $[G \stackrel{Ad}{\to} Aut(G)]$. The morphism $AUT(G) \to Out(G)$ is a fibration hence its homotopy fiber is, up to equivalence, the ordinary fiber, which is the crossed module $(G \stackrel{Ad}{\to} Inn(G))$, where $Inn(G) \subset Aut(G)$ is the group of inner automorphisms. This is equivalent to $(Z(G) \to 1)$, where $Z(G)$ is the ordinary center of $G$, and this is the crossed module corresponding to $\mathbf{B}Z(G)$.

Last revised on July 31, 2018 at 05:30:10. See the history of this page for a list of all contributions to it.