# Set is Clopen iff Boundary is Empty

## Theorem

Let $T$ be a topological space, and let $H \subseteq T$.

Then $H$ is both closed and open in $T$ if and only if:

- $\partial H = \varnothing$

where $\partial H$ is the boundary of $H$.

## Proof

From Set is Open iff Disjoint from Boundary we have that:

- $H$ is open in $T$ if and only if $\partial H \cap H = \varnothing$

From Set is Closed iff it Contains its Boundary we have that:

- $H$ is closed in $T$ if and only if $\partial H \subseteq H$

From the above it follows that $\partial H = \varnothing$ if and only if $H$ is closed and $H$ is open.

$\blacksquare$

## Sources

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