Definition:T4 Space

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

$\left({X, \vartheta}\right)$ is a $T_4$ space iff:
 * $\left({X, \vartheta}\right)$ is a normal space
 * $\left({X, \vartheta}\right)$ is a Fréchet ($T_1$) space.

That is:


 * $\forall A, B \in \complement \left({\vartheta}\right), A \cap B = \varnothing: \exists U, V \in \vartheta: A \subseteq U, B \subseteq V$


 * $\forall x, y \in X$, both:
 * $\exists U \in \vartheta: x \in U, y \notin U$
 * $\exists V \in \vartheta: y \in V, x \notin V$

Variants of Name
Earlier (pre-1970) treatment of this subject tends to refer to this as a normal space, and what we define as a normal space as a $T_4$ space.