Definition:Closed Ball/Metric Space/Center

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

Let $a \in A$.

Let $\map {B^-_\epsilon} a$ be the closed $\epsilon$-ball of $a$.

In $\map {B^-_\epsilon} a$, the value $a$ is referred to as the center of the closed $\epsilon$-ball.