Given $ a\in R^n - A $, the set $A$ is open if, and only if, $a$ is an point isolated the set the bonder $ \partial X $.

Given $ a\in R^n - A $, the set $A$ is open if, and only if, $a$ is an
point isolated the set the bonder $ \partial X $.

Let $A \subset R^n $ be an open set with $n \ge 2$. Given $ a\in R^n - A
$, the set $A$ is open if, and only if, $a$ is an point isolated the set
the bonder $ \partial X $.