The simplest Hahn-Banach extension theorem in Hilbert space $X$ avoids the use of the axiom of choice by virtue of the Riesz representation theorem. But what about the version of the theorem where the sought-for linear functional is required to remain below a given sub-linear or convex function? Also, to which extent can we separate 2 disjoint convex sets by a hyperplane without Zorn? Can we assert that any hyperplane $H\subset X$ has a translate that is tangent (*) to a given bounded closed convex subset $C\subset X$?

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(*) "tangent" means that the linear bounded function whose kernel represents the hyperplane is non-negative on $C$ but not strictly positive.

  • $\begingroup$ To separate a point $x$ from a closed convex set $C$, one can take the best approximation $y\in C$ to $x$ and a hyperplane perpendicular to $x-y$. $\endgroup$ – Dirk Werner Feb 27 at 15:32
  • $\begingroup$ And similarly one can find nearest points between closed convex set and a disjoint compact convex body, but that's about "the best" that I'm aware of. $\endgroup$ – Thibaut Demaerel Feb 27 at 17:01

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