# Closed Ball in Metric Space is Closed Neighborhood

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## Theorem

Let $M = \struct {A, d}$ be a metric space.

Let $x \in A$.

Let $\epsilon \in \R_{>0}$.

Let $\map {B_\epsilon^-} x$ be the closed $\epsilon$-ball of $x$ in $M$.

Then $\map {B_\epsilon^-} x$ is a closed neighborhood of $x$ in $M$.

## Proof

From Closed Ball contains Smaller Open Ball:

- $\map {B_\epsilon} x \subseteq \map {B_\epsilon^-} x$

where $\map {B_\epsilon} x$ denotes the open $\epsilon$-ball of $x$ in $M$.

Hence $\map {B_\epsilon^-} x$ is a neighborhood of $x$ by definition.

From Closed Ball is Closed in Metric Space:

- $\map {B_\epsilon^-} x$ is closed in $M$

It follows that $\map {B_\epsilon^-} x$ is a closed neighborhood of $x$ in $M$ by definition.

$\blacksquare$