Definition:Separable Space/Normed Vector Space

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

Let $Y \subseteq X$ be a subset of $X$.

Let $Y$ be (everywhere) dense in $X$.

Let $Y$ be a countable set:


 * $Y = \set {y_1, y_2, \ldots}$

Suppose:


 * $\forall x \in X : \forall \epsilon \in \R : \epsilon > 0 : \exists y_{n \mathop \in \N} \in Y : \norm {y_n - x} < \epsilon$

Then $X$ is separable.