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Separation of an Open Convex Set from a Disjoint Convex Set in a Separable Real Normed Space

The first geometric form of the Hahn–Banach theorem: in a separable real normed space, two disjoint nonempty convex sets, one of them open, are separated by a closed hyperplane F = α, with strict inequality on the open set.

Statement

In the setting of Separable Real Normed Spaces and Hahn–Banach: Notation, let XX be a separable real normed space, and let A,B⊆XA,B\subseteq X be nonempty, disjoint and convex, with AA open.

Separation. There are an element FF of the dual space X∗X^{*}, not identically zero, and a real number α\alpha with

F(a)<α≤F(b)for every a∈A and every b∈B.F(a)<\alpha\le F(b)\qquad\text{for every }a\in A\text{ and every }b\in B.

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