T0 Space

Definition

A Topological Space $(X,\mathcal{T})$ is $T_0$ space if $\forall x,y\in X\text{s.t.}x\ne y,\exists {\mathcal{N}}_x\text{s.t.}y\notin {\mathcal{N}}_x\text{or}\exists {\mathcal{N}}_y\text{s.t.}x\notin {\mathcal{N}}_y$