Topological Subspace is Topological Space

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

Let $H \subseteq X$ be a non-empty subset of $X$.

Let $\tau_H = \left\{{U \cap H: U \in \tau}\right\}$.

Then $\left({H, \tau_H}\right)$ is a topological space.

Also see

 * Topological Subspace