Normal Space is Tychonoff Space

Theorem
Let $\left({X, \vartheta}\right)$ be a $T_4$ space.

Then $\left({X, \vartheta}\right)$ is also a $T_{3 \frac 1 2}$ (Tychonoff) space.

Proof
Let $T = \left({X, \vartheta}\right)$ be a $T_4$ space.

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

Let $F$ be any closed set in $T$, and let $y \in \complement_X \left({F}\right)$, that is, $y \in X$ such that $y \notin F$.

As $T$ is a Fréchet ($T_1$) space it follows from Equivalent Definitions for $T_1$ Space that $\left\{{y}\right\}$ is closed.

As $T = \left({X, \vartheta}\right)$ is a normal space, we have that for any two disjoint closed sets $A, B \subseteq X$ there exists an Urysohn function for $A$ and $B$.

But $F$ and $\left\{{y}\right\}$ are disjoint closed sets.

So there exists an Urysohn function for $F$ and $\left\{{y}\right\}$.

This is the definition of a completely regular space.

Next we note that as $T$ is a Fréchet ($T_1$) space, from $T_1$ Space is $T_0$ Space $T$ is a Kolmogorov ($T_0$) space.

So $T$ is both completely regular and Kolmogorov ($T_0$), and so by definition it is a $T_{3 \frac 1 2}$ (Tychonoff) space.