Superset of Neighborhood in Metric Space is Neighborhood
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Theorem
Let $M = \struct {A, d}$ be a metric space.
Let $a \in A$ be a point in $M$.
Let $N$ be a neighborhood of $a$ in $M$.
Let $N \subseteq N' \subseteq A$.
Then $N'$ is a neighborhood of $a$ in $M$.
Proof
By definition of neighborhood:
- $\exists \epsilon \in \R_{>0}: \map {B_\epsilon} a \subseteq N$
where $\map {B_\epsilon} a$ is the open $\epsilon$-ball of $a$ in $M$.
By Subset Relation is Transitive:
- $\map {B_\epsilon} a \subseteq N'$
The result follows by definition of neighborhood of $a$.
$\blacksquare$
Sources
- 1975: Bert Mendelson: Introduction to Topology (3rd ed.) ... (previous) ... (next): Chapter $2$: Metric Spaces: $\S 4$: Open Balls and Neighborhoods: Theorem $4.8: \ N \, 3$