# Urysohn's lemma

Let $X$ be a Normal space (?) (i.e., a topological space that is T1 and where disjoint closed subsets can be separated by disjoint open subsets). Suppose $A,B$ are disjoint closed subsets of $X$. Then, there exists a continuous function $f:X \to [0,1]$ such that $f(a) = 0$ for all $a \in A$, and $f(b) = 1$ for all $b \in B$.