Quasicomponent is Intersection of Clopen Sets

Theorem
Let $T = \left({S, \tau}\right)$ be a topological space.

Let $p \in S$.

Then the quasicomponent containing $p$ equals the intersection of all sets which are both open and closed which contain $p$.

Proof
Let $C$ be the quasicomponent of $p$.

Let $Q$ be the set of clopen sets containing $p$ that are not equal to $S$.

First let $x \in C$.

Let $U \in Q$.

By definition of $Q$:
 * $p \in U$

Let $V = \complement_S \left({U}\right)$ be the complement of $U$ relative to $S$.

By Complement of Clopen Space is Clopen $V$ is also a clopen set of $T$.

By Clopen Set and Complement form Separation $U$ and $V$ separate $S$.

By the definition of quasicomponent:
 * $x \in U$

As this holds for all such $U$:
 * $x \in \bigcap Q$

As this holds for all such $x$:
 * $C \subseteq Q$

Now instead let $x \in \bigcap Q$.

Let $U$ and $V$ separate $S$.

Let $p \in U$.

Then $U \in Q$.

Thus $x \in U$.

Thus $x$ lies on the same side as $p$ of any separation of $X$.

So $x \in C$.