Inner Limit in Normed Spaces by Open Balls

Theorem
Let $\left \langle{C_n}\right \rangle_{n \in \N}$ be a sequence of sets in a normed space $\left(\mathcal{X},\|\cdot\|\right)$. Then the inner limit of $\left \langle{C_n}\right \rangle_{n \in \N}$ is:


 * $\displaystyle \liminf_n \ C_n = \left\{{x: \forall \varepsilon > 0: \exists N \in \mathcal{N}_\infty: \forall n \in N: x \in C_n + \varepsilon \mathcal B}\right\}$

where $\mathcal{B}$ stands for the open unit ball of the space.

Proof
The proof is an immediate result of Inner Limit in Hausdorff Space by Open Neighborhoods since the arbitrary open sets can be here replaced by open balls.