The open ball in a metric space $(X,d)$ with radius $\epsilon$ centered at a, is denoted $B(a,\epsilon )$. Formally $B(a,\epsilon )=\{x\in X:d(a,x)<\epsilon \}$

Let $(X,d)$ be a metric space. We say a set $A\subset X$ is open if for every $x\in A{\text{ }}\exists \epsilon >0$ such that $B(x,\epsilon )\subset A$.

We say a set $B\subset X$ is closed if $X\backslash B$ is open.