Neighborhood in Metric Space has Subset Neighborhood

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 $M$.

Then there exists a neighborhood $N'$ of $a$ such that:
 * $(1): \quad N' \subseteq N$
 * $(2): \quad N'$ is a neighborhood of each of its points.

Proof
By definition of neighborhood:


 * $\exists \epsilon \in \R_{>0}: \map {B_\epsilon} a \subseteq N$

By Open Ball is Neighborhood of all Points Inside, $N' = \map {B_\epsilon} a$ fulfils the conditions of the statement.