Definition 5.1.2 (Closed Set).label Let $(X, \topo)$ be a topological space, then $A \subset X$ is closed if $A^{c} \in \topo$.
Definition 5.1.2 (Closed Set).label Let $(X, \topo)$ be a topological space, then $A \subset X$ is closed if $A^{c} \in \topo$.