A set A ⊂ X {\displaystyle A\subset X} , where ( X , O ) {\displaystyle (X,O)} is some topological space, is said to be closed if X − A = { x ∈ X ∣ x ∉ A } {\displaystyle X-A=\{x\in X\mid x\notin A\}} , the complement of X {\displaystyle X} in A {\displaystyle A} , is an open set
Analysis
