Local Basis of Open Subspace iff Local Basis
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Theorem
Let Let $T = \struct{S, \tau}$ be a topological space.
Let $U \subseteq S$ be an open subset
Let $\tau_U$ denote the subspace topology on $U$.
Let $s \in U$.
Let $\BB \subseteq \powerset U$.
Then:
- $\BB$ is a local basis of $s$ in $\struct{U, \tau_U}$ if and only if $\BB$ is a local basis of $s$ in $\struct{S, \tau}$
Proof
Let $\map \BB s$ denote the set of open sets containing $s$ in $\struct{S, \tau}$
Let $\map \CC s$ denote the set of open sets containing $s$ in $\struct{U, \tau_U}$
From Open Set in Open Subspace:
- $\BB \subseteq \map \BB s$ if and only if $\BB \subseteq \map \CC s$
Necessary Condition
Let $\BB$ be a local basis of $s$ in $\struct{U, \tau_U}$.
Let $W \in \map \BB s$.
By definition of subspace topology:
- $W' = W \cap U \in \tau_U$
By definition of local basis:
- $\exists V \in \BB : x \in V \subseteq W'$
From Subset Relation is Transitive:
- $V \subseteq W$
We have:
- $\exists V \in \BB : x \in V \subseteq W$
It follows that:
- $\BB$ is a local basis of $s$ in $\struct{S, \tau}$
$\Box$
Sufficient Condition
Let $\BB$ be a local basis of $s$ in $\struct{S, \tau}$.
Let $W \in \map \CC s$.
From Open Set in Open Subspace:
- $W \in \map \BB s$.
Hence:
- $\exists V \in \BB : x \in V \subseteq W$
It follows that:
- $\BB$ is a local basis of $s$ in $\struct{U, \tau_U}$
$\blacksquare$