# Definition:Exterior (Topology)/Definition 1

## Definition

Let $T$ be a topological space.

Let $H \subseteq T$.

The exterior of $H$ is the complement of the closure of $H$ in $T$.

## Notation

The exterior of $H$ can be denoted:

$\map {\mathrm {Ext} } H$
$H^e$

The first is regarded by some as cumbersome, but has the advantage of being clear.

$H^e$ is neat and compact, but has the disadvantage of being relatively obscure.

On $\mathsf{Pr} \infty \mathsf{fWiki}$, the notation of choice is $H^e$.