T4 Property Preserved in Closed Subspace

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

Let $T_K$ be a subspace of $T$ such that $K$ is closed in $T$.

If $T$ is a $T_4$ space then $T_K$ is also a $T_4$ space.

That is, the property of being a $T_4$ space is weakly hereditary.

Proof
Let $T = \struct {S, \tau}$ be a $T_4$ space.

Then:
 * $\forall A, B \in \map \complement \tau, A \cap B = \O: \exists U, V \in \tau: A \subseteq U, B \subseteq V, U \cap V = \O$

That is, for any two disjoint closed sets $A, B \subseteq S$ there exist disjoint open sets $U, V \in \tau$ containing $A$ and $B$ respectively.

We have that the set $\tau_K$ is defined as:
 * $\tau_K := \set {U \cap K: U \in \tau}$

where $K$ is closed in $T$.

Let $A, B \subseteq K$ be closed in $K$ such that $A \cap B = \O$.

From Intersection with Subset is Subset‎ we have that $A \subseteq K \iff A \cap K = A$.

As $K$ is itself closed in $T$, it follows that so is $A \cap K = A$ from Topology Defined by Closed Sets.

Similarly, so is $B \cap K = B$.

Because $T$ is a $T_4$ space, we have that:
 * $\exists U, V \in \tau: A \subseteq U, B \subseteq V, U \cap V = \O$

As $A, B \subseteq K$ such that we have that:
 * $A \subseteq U \cap K, B \subseteq V \cap K: \paren {U \cap K} \cap \paren {V \cap K} = \O$

From the definition of topological subspace, both $U \cap K$ and $V \cap K$ are open in $K$.

Thus the $T_4$ axiom is satisfied in $K$.

Also see

 * $T_4$ Property is not Hereditary.

The above proof hinges on the fact that as $K$ is closed in $T$, then $A \cap K = A$ and $B \cap K = B$ are also closed in $T$.

If $K$ is not closed in $T$, there is no such guarantee of closedness of $A$ and $B$.