Boundary of Subset of Indiscrete Space

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

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

Then:
 * $\partial H = S$

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

If $H = \varnothing$ or $H = S$ then $\partial H = \varnothing$.

Proof
From Closure of Subset of Indiscrete Space, $H^- = S$, where $H^-$ denotes set closure.

From Interior of Subset of Indiscrete Space, $H^\circ = \varnothing$, where $H^\circ$ denotes set interior.

By definition:
 * $\delta H = H^- \setminus H^\circ = S \setminus \vartheta = S$

From Open and Closed Sets in Topological Space both $\varnothing$ and $S$ are both closed and open in $T$.

So if $H = \varnothing$ or $H = S$ then $H = H^\circ = H^-$ from Interior of Open Set and Closed Set Equals its Closure.

So $\delta H = H \setminus H = \varnothing$.