Monotonically normal implies normal

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This article gives the statement and possibly, proof, of an implication relation between two topological space properties. That is, it states that every topological space satisfying the first topological space property (i.e., monotonically normal space) must also satisfy the second topological space property (i.e., normal space)
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Any monotonically normal space is a normal space.

Definitions used

Monotonically normal space

Further information: monotonically normal space

A topological space X is termed monotonically normal if it is a T_1 space and there exists an operator (called a monotone normality operator) G from pairs of disjoint closed subsets (A,B) to open subsets such that:

  • For all disjoint closed subsets A,B, G(A,B) is an open subset containing A and whose closure is disjoint from B.
  • For closed subsets A,A',B,B', if A \subseteq A', B' \subseteq B, with A,B disjoint and A',B' disjoint, we have G(A,B) \subseteq G(A',B').

Normal space

Further information: normal space

A topological space X is termed normal if it is a T_1 space and, for any two disjoint closed subsets A and B, there exist disjoint open subsets U and V such that U contains A and V contains B.


Given: A monotonically normal space X with a monotone normality operator G.

To prove: X is a normal space.

Proof: The T_1-space part follows by definition. For the other part, note that U = G(A,B) and V = X \setminus \overline{G(A,B)} are disjoint open subsets containing A and B respectively.