Definition:Open Set/Normed Vector Space

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

Let $U \subset X$.

Then $U$ is an open set in $V$ :


 * $\forall x \in U: \exists \epsilon \in \R_{>0}: \map {B_\epsilon} x \subset U$

where $\map {B_\epsilon} x$ is the open $\epsilon$-ball of $x$.