Topological Subspace is Topological Space/Proof 2
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Theorem
Let $\struct {X, \tau}$ be a topological space.
Let $H \subseteq X$ be a non-empty subset of $X$.
Let $\tau_H = \set {U \cap H: U \in \tau}$ be the subspace topology on $H$.
Then the topological subspace $\struct {H, \tau_H}$ is a topological space.
Proof
Follows directly from Subspace Topology is Initial Topology with respect to Inclusion Mapping.
$\blacksquare$