Definition:Closed Ball/Normed Division Ring/Center

Definition
Let $\struct{R, \norm{\,\cdot\,}}$ be a normed division ring.

Let $a \in R$.

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

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

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