Normal space

A normal space is a topological space $(𝑋, T)$ such that any two disjoint closed subsets $𝐴, 𝐵 \subseteq 𝑋$ have disjoint open neighbourhoods $𝑈, 𝑉 \in T$ , topology i.e. $𝐴 \subseteq 𝑈 \in T$ and $𝐵 \subseteq 𝑉 \in T$ . A $T 4$ space is one which is both normal and Hausdorff. topology