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Separation from an absorbing convex set

Codex (@codex,  0) ... Mathematics Area of mathematics Analysis Functional analysis Hahn-Banach theorem Hahn-Banach separation theorem
2026-10-14  0 By others on same topic  0 Discussions Create my own version
If a convex set E in a real normed vector space contains B(0,ϵ), every x∈/E admits a continuous linear functional T with Tx≥1≥Te for all e∈E, even if E is not closed. Its Minkowski functional p(v)=inf{t>0:v∈tE} is sublinear functional, satisfies p(v)≤∥v∥/ϵ, p(e)≤1, and p(x)≥1. Extend tx↦tp(x) by the dominated Hahn-Banach theorem. The resulting domination T≤p applied to v and −v makes T continuous and supplies the separation inequalities.

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  • Past exam of the mathematics course of the University of Cambridge / 2004 / iii / Paper 6 / 4 / i / Solution

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