Definition:Perfectly Normal Space

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

$\left({S, \tau}\right)$ is a perfectly normal space :
 * $\left({S, \tau}\right)$ is a perfectly $T_4$ space
 * $\left({S, \tau}\right)$ is a $T_1$ (Fréchet) space.

That is:


 * Every closed set in $T$ is a $G_\delta$ set.


 * $\forall x, y \in S$, both:
 * $\exists U \in \tau: x \in U, y \notin U$
 * $\exists V \in \tau: y \in V, x \notin V$