Subspace Topology is Initial Topology with respect to Inclusion Mapping
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Theorem
Let $\struct {X, \tau}$ be a topological space.
Let $Y$ be a non-empty subset of $X$.
Let $\iota: Y \to X$ be the inclusion mapping.
Let $\tau_Y$ be the initial topology on $Y$ with respect to $\iota$.
Then $\struct {Y, \tau_Y}$ is a topological subspace of $\struct {X, \tau}$.
That is:
- $\tau_Y = \set {U \cap Y: U \in \tau}$
Proof
By Initial Topology with respect to Mapping equals Set of Preimages, it follows that:
- $\tau_Y = \set {\iota^{-1} \sqbrk U: U \in \tau}$
From Preimage of Subset under Inclusion Mapping, we have:
- $\forall S \subseteq X: \iota^{-1} \sqbrk S = S \cap Y$
Hence the result.
$\blacksquare$