Definition:Open Ball/Normed Vector Space

Definition
Let $\struct{X, \norm {\,\cdot\,} }$ be a normed vector space.

Let $x \in X$.

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

The open $\epsilon$-ball of $x$ in $\struct {X, \norm {\,\cdot\,} }$ is defined as:


 * $\map {B_\epsilon} x = \set {y \in X: \norm{x - y} < \epsilon}$