# nLab delooping

### Context

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

(∞,1)-category theory

## Models

#### Stabe homotopy theory

stable homotopy theory

# Contents

## Idea

The delooping of an object $A$ is, if it exists, a uniquely pointed object $\mathbf{B} A$ such that $A$ is the loop space object of $\mathbf{B} A$:

$A \simeq \Omega(\mathbf{B} A)$

In particular, if $A = G$ is a group then its delooping

• in the context Top is the classifying space $\mathcal{B}G$

• in the context ∞-Grpd is the one-object groupoid $\mathbf{B}G$.

Under the homotopy hypothesis these two objects are identified: the geometric realization of the groupoid $\mathbf{B}G$ is the classifying space $\mathcal{B}G$:

$|\mathbf{B}G| \simeq \mathcal{B}G \,.$

## Definition

Loop space objects are defined in any (∞,1)-category $\mathbf{C}$ with homotopy pullbacks: for $X$ any pointed object of $\mathbf{C}$ with point ${*} \to X$, its loop space object is the homotopy pullback $\Omega X$ of this point along itself:

$\array{ \Omega X &\to& {*} \\ \downarrow && \downarrow \\ {*} &\to& X } \,.$

Conversely, if $A$ is given and a homotopy pullback diagram

$\array{ A &\to& {*} \\ \downarrow && \downarrow \\ {*} &\to& \mathbf{B}A }$

exists, with the point ${*} \to \mathbf{B} A$ being essentially unique, by the above $A$ has been realized as the loop space object of $\mathbf{B} A$

$A = \Omega \mathbf{B} A$

and we say that $\mathbf{B} A$ is the delooping of $A$.

See the section delooping at groupoid object in an (∞,1)-category for more.

## Remarks

If $\mathbf{C}$ is even a stable (∞,1)-category then all deloopings exist and are then also denoted $\Sigma A$ and called the suspension of $A$.

## Characterization of deloopable objects

In section 6.1.3 of

a definition of groupoid object in an (infinity,1)-category $\mathbf{C}$ is given as a homotopy simplicial objects, i.e. a (infinity,1)-functor

$C : \Delta^{op} \to \mathbf{C}$
$\cdots C_2 \stackrel{\to}\rightrightarrows C_1 \rightrightarrows C_0$

satisfying certain conditions (prop. 6.1.2.6) which are such that if $C_0 = {*}$ is the point we have an internal group in a homotopical sense, given by an object $C_1$ equipped with a coherently associative multiplication operation $C_1 \times C_1 \to C_1$ generalizing that of Stasheff H-space from the $(\infty,1)$-category Top to arbitrary $(\infty,1)$-categories.

Lurie calls the groupoid object $C$ an effective groupoid object in an (infinity,1)-category precisely if it arises as the delooping, in the above sense, of some object $\mathbf{B}C$.

One of the characterizing properties of an (infinity,1)-topos is that every groupoid object in it is effective.

This is the analog of Stasheff’s classical result about H-spaces.

See the remark at the very end of section 6.1.2 in HTT.

## Examples

### Topological loop spaces

For $C =$ Top the (infinity,1)-category of topological spaces, a space is deloopable if it is an A-infinity-space and hence homotopy equivalent to a loop space.

### Delooping of a group to a groupoid

Let $G$ be a group regarded as a discrete groupoid in the (∞,1)-topos ∞Grpd of ∞-groupoids.

Then $\mathbf{B} G$ exists and is, up to equivalence, the groupoid

• with a single object $\bullet$,

• with $Hom_{\mathbf{B} G}(\bullet, \bullet) = G$, or equivalently $Aut_{\mathbf{B}G}(\bullet) = G$,

• and with composition of morphisms in $\mathbf{B} G$ being given by the product operation in the group.

$\mathbf{B} G = \{ \bullet \stackrel{g}{\to} \bullet | g \in G\}$

or

$\mathbf{B}G = \{ \bullet \righttoleftarrow g \;|\; g \in G \}$

to emphasize that there is really only a single object.

Notice how the homotopy pullback works in this simple case:

the universal 2-cell $\eta$

$\array{ G &\to& {*} \\ \downarrow &\Downarrow^{\eta}& \downarrow \\ {*} &\to& \mathbf{B}G }$

filling this 2-limit diagram is the natural transformation from the constant functor

$G \to {*} \to \mathbf{B}G$

to itself, whose component map

$\eta : Obj(G) \to Mor(\mathbf{B}G)$

is just the identity map, using that $Obj(G) = G$ and $Mor(\mathbf{B}G) = G$.

Revised on February 3, 2013 15:43:11 by Tim Porter (95.147.236.249)