Equivalence of Definitions of Boundary

Theorem
Let $T = \struct {S, \tau}$ be a topological space.

Let $H \subseteq S$.

Proof
Let $x \in S$.

By definition of set difference:
 * $x \in H^- \setminus H^\circ$ $x \in H^-$ and $x \notin H^\circ$

By definition of the closure:
 * $x \in H^-$ every neighborhood $N$ of $x$ satisfies $H \cap N \neq \O$

Lemma
So $x \in H^- \setminus H^\circ$ every neighborhood $N$ of $x$ satisfies:
 * $H \cap N \neq \O$

and
 * $\paren{S \setminus H} \cap N \neq \O$