Definition:Exterior (Topology)/Definition 2

Definition

Let $T$ be a topological space.

Let $H \subseteq T$.

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

Notation

The exterior of $H$ can be denoted:

$\operatorname{Ext} \left({H}\right)$

$H^e$

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 $H^e$.

Also see

• Equivalence of Definitions of Exterior

Sources

• 1970: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology ... (previous) ... (next): $\text{I}: \ \S 1$: Closures and Interiors