# nLab category of open subsets

### Context

#### Topology

topology

algebraic topology

## Examples

#### Topos Theory

Could not include topos theory - contents

# Category of open subsets

## Definition

Given a topological space $X$, the category of open subsets $Op(X)$ of $X$ is the category whose

• objects are the open subsets $U \hookrightarrow X$ of $X$;

• morphisms are the inclusions $\array{ V &&\hookrightarrow && U \\ & \searrow && \swarrow \\ && X }$ of open subsets into each other.

## Properties

• The category $Op(X)$ is a poset, in fact a frame (dually a locale): it is the frame of opens of $X$.

• The category $Op(X)$ is naturally equipped with the structure of a site, where a collection $\{U_i \to U\}_i$ of morphisms is a cover precisely if their union in $X$ equals $U$:

$\bigcup_i U_i = U .$

The category of sheaves on $Op(X)$ equipped with this site structure is usually written

$Sh(X) := Sh(Op(X)) \,.$

Revised on August 31, 2012 14:27:37 by Urs Schreiber (89.204.139.6)