Interior of Subset of Indiscrete Space

Theorem
Let $T = \left({S, \left\{{\varnothing, S}\right\}}\right)$ be an indiscrete topological space.

Let $H \subset S$ (that is, let $H$ be a proper subset of $T$).

Then:
 * $H^\circ = H^{\circ -} = H^{\circ - \circ} = \varnothing$

where:
 * $H^\circ$ denotes the interior of $H$


 * $H^-$ denotes the closure of $H$.

Proof
As $H \subset S$, it follows that $H \ne T$.

So the only open subset of $H$ is $\varnothing$.

So by definition:
 * $H^\circ = \varnothing$

From Empty Set is Closed in Topological Space, $\varnothing$ is closed in $T$.

From Closed Set Equals its Closure:
 * $\varnothing^- = \varnothing$

From Empty Set is Element of Topology, $\varnothing$ is open in $T$.

From Interior of Open Set:
 * $\varnothing^\circ = \varnothing$

The result follows.