Separation Properties Preserved in Subspace

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

Let $T_H$ be a subspace of $T$.

If $T$ has one of the following properties then $T_H$ has the same property:


 * $T_0$ (Kolmogorov) Property


 * $T_1$ (Fréchet) Property


 * $T_2$ (Hausdorff) Property


 * $T_{2 \frac 1 2}$ (Completely Hausdorff) Property


 * $T_3$ Property


 * $T_{3 \frac 1 2}$ Property


 * $T_5$ Property

Corollary
If $T$ has one of the following properties then $T_H$ has the same property:


 * Regular Property


 * Tychonoff (Completely Regular) Property


 * Completely Normal Property

Note
Of all the separation axioms, the $T_4$ axiom differs from the others.

It does not necessarily hold that a subspace of a $T_4$ space is also a $T_4$ space, unless that subspace is closed.