Underlying Set of Topological Space is Clopen

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

Then the underlying set $S$ of $T$ is both open and closed in $T$.

Proof
From the definition of topology, $S$ is open in $T$.

From Underlying Set of Topological Space is Closed $S$ is closed in $T$.

Hence the result.