Definition:Closed Ball/Metric Space/Radius

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

Let $a \in A$.

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

The value $\epsilon$ is referred to as the radius of $\map { {B_\epsilon}^-} a$.