T0
Description: If $a, b \in X$, there exists an open set $U$ such that either $a \in U$ and $b \notin U$, or $b \in U$ and $a \notin U$.
Description: If $a, b \in X$, there exists an open set $U$ such that either $a \in U$ and $b \notin U$, or $b \in U$ and $a \notin U$.