Let $C$ be an closed subset of the Banach space $X$. I am wondering whether the following statements are equivalent for any Banach space:
$O$ is an open set containing $C$.
For every $x\in \partial C$ and every direction $y$, there exists $\epsilon >0$ such that $x+\epsilon y$ is in $O$.
For every sequence $x_n$ converging to $x\in \partial C$, there is an $N$ such that $x_n\in O$, $n\ge N$.
3 does not imply 1.
Let X = R, C = {1}, O = [0,2].
O is a nhood of C could get you better results.