Definition

A Topological Space $(X, T)$ is perfectly normal ( $T_{6}$ ) space if it satisfies T4 Axiom and every Closed Set $(X ∖ A) \in T$ is a G-delta Set. $\forall (X ∖ A) \in T, \exists {U_{i}}_{i \in N} s.t. U_{i} \in T and ⋂_{i \in N} U_{i} = A$