Interior of Subset of Indiscrete Space

Theorem
Let $T = \struct {S, \set {\O, S} }$ be an indiscrete topological space.

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

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

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


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

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

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

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

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

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

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

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

The result follows.