Topological Space is Open Neighborhood of Subset
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Theorem
Let $T = \left({S, \tau}\right)$ be a topological space.
Let $H \subseteq S$ be a subset of $S$.
Then $S$ is an open neighborhood of $H$.
Proof
From Underlying Set of Topological Space is Clopen, $S$ is open in $T$.
By hypothesis, $H \subseteq S$.
The result follows from Open Superset is Open Neighborhood.
$\blacksquare$