Characterization of T3 Space

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


 * $(1) \quad T$ is a $T_3$ space
 * $(2) \quad \forall F \subseteq S : S \setminus F \in \tau, y \in S \setminus F : \exists W \in \tau : y \in W, W^- \cap F = \O$
 * $(3) \quad \forall U \in S, y \in U : \exists V \in \tau : y \in V, V^- \subseteq U$

where $A^-$ denotes the closure of $A$ in $T$

Statement 1 implies Statement 2
Let $T$ be a $T_3$ space.

Let $F \subseteq S : S \setminus F \in \tau$.

Let $y \in S \setminus F$.

By definition of $T_3$ space:
 * $\exists W, V \in \tau : y \in W, F \subseteq V : W \cap V = \O$

From Subset of Set Difference iff Disjoint Set:
 * $W \subseteq S \setminus V$

From Closure of Subset of Closed Set of Topological Space is Subset:
 * $W^- \subseteq S \setminus V$

From Set Difference with Subset is Superset of Set Difference:
 * $S \setminus V \subseteq S \setminus F$

From Subset Relation is Transitive:
 * $W^- \subseteq S \setminus F$

From Subset of Set Difference iff Disjoint Set:
 * $W^- \cap F = \O$

The result follows.

Statement 2 implies Statement 3
Let $T$ satisfy:
 * $\forall F \subseteq S : S \setminus F \in \tau, y \in S \setminus F : \exists W \in \tau : y \in W, W^- \cap F = \O$

Statement 3 implies Statement 1
Let $T$ satisfy:
 * $\forall U \in S, y \in U : \exists V \in \tau : y \in V, V^- \subseteq U$