# nLab essentially small (infinity,1)-category

### Context

#### Compact objects

objects $d\in C$ such that $C\left(d,-\right)$ commutes with certain colimits

## Relative version

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

(∞,1)-category theory

# Contents

## Idea

The notion of essentially small $\left(\infty ,1\right)$-category is the generalization of the notion of essentially small category from category theory to (∞,1)-category theory.

## Definitions

###### Definition

A quasi-category $C$ is essentially $\kappa$-small for some regular cardinal $\kappa$ if

1. the collection of equivalence classes in $C$ is $\kappa$-small;

2. for every morphism $f:x\to y$ in $C$ the homotopy sets of the hom ∞-groupoid at $f$ (that is, the sets ${\pi }_{i}\left({\mathrm{Hom}}^{R}\left(x,y\right),f\right)$) are $\kappa$-small.

$C$ is essentially small if the above conditions hold “absolutely,” i.e. with ”$\kappa$-small” replaced by “small.”

This appears as HTT, def. 5.4.1.3, prop. 5.4.1.2.

In the presence of the regular extension axiom (which follows from the axiom of choice), essential smallness is equivalent to being essentially $\kappa$-small for some small regular cardinal $\kappa$.

## Properties

###### Proposition

Let $C$ be an (∞,1)-category and $\kappa$ an uncountable regular cardinal. The following are equivalent:

1. $C$ is $\kappa$-small.

2. $C$ is a $\kappa$-compact object in (∞,1)Cat.

3. $C$ is equivalently given by a quasi-category whose underlying simplicial set is a $\kappa$-small set.

This is HTT, prop. 5.4.1.2

The analogous statement holds for ∞-groupoids.

###### Proposition

For $X$ an ∞-groupoid and $\kappa$ an uncountable regular cardinal, the following are equivalent

1. For each object $x\in C$ the homotopy sets ${\pi }_{n}\left(X,x\right)$ are $\kappa$-small sets.

2. $X$ is presented by a $\kappa$-small simplicial set/Kan complex.

3. $X$ is a $\kappa$-compact object in ∞Grpd.

This is (HTT, corollary 5.4.1.5).

## References

This is the topic of section 5.4.1 of

Revised on December 11, 2011 07:21:12 by Urs Schreiber (91.221.145.249)