# Definition:Sphere/Metric Space

## Definition

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

Let $a \in A$.

Let $\epsilon \in \R_{>0}$ be a strictly positive real number.

The $\epsilon$-sphere of $a$ in $M$ is defined as:

$\map {S_\epsilon} a = \set {x \in A: \map d {x, a} = \epsilon}$

In $S_\epsilon \paren{a}$, the value $\epsilon$ is referred to as the radius of the $\epsilon$-sphere.
In $S_\epsilon \paren{a}$, the value $a$ is referred to as the center of the $\epsilon$-sphere.