# nLab subobject in an (infinity,1)-category

Contents

### Context

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

(∞,1)-category theory

Background

Basic concepts

Universal constructions

Local presentation

Theorems

Extra stuff, structure, properties

Models

# Contents

## Definition

Let $C$ be an (∞,1)-category and $X \in C$ an object.

###### Definition

A subobject of $X$ is a 1-monomorphism $K \hookrightarrow X$ into $X$.

The (∞,1)-category $Sub(X)$ of subobjects of $X$ is the $(-1)$-truncation of the slice-(∞,1)-category of $C$ over $X$

$Sub(X) := \tau_{-1} C_{/X} \,.$

This is the category whose objects are monomorphisms $U \hookrightarrow X$ in $C$ and whose morphisms are 2-morphisms

$\array{ U_1 &&\to&& U_2 \\ & \searrow &\swArrow& \swarrow \\ && X }$

in $C$.

## Properties

###### Proposition

$Sub(X)$ is a (0,1)-category (a poset).

This appears for instance in (Lurie, section 6.2).

###### Proposition

If $C$ is a locally presentable (∞,1)-category then $Sub(X)$ is a small category.

• subobject

• subobject in an $(\infty,1)$-category