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Journal of Convex Analysis 21 (2014), No. 4, 1065--1084
Copyright Heldermann Verlag 2014



Extension of Continuous Convex Functions from Subspaces I

Carlo Alberto De Bernardi
Dipartimento di Matematica, UniversitÓ degli Studi, Via C. Saldini 50, 20133 Milano, Italy
carloalberto.debernardi@gmail.com

Libor Veselř
Dipartimento di Matematica, UniversitÓ degli Studi, Via C. Saldini 50, 20133 Milano, Italy
libor.vesely@unimi.it



[Abstract-pdf]

Let $X$ be a topological vector space, $Y\subset X$ a subspace, and $A\subset X$ an open convex set containing $0$. We are interested in the extendability of a continuous convex function $f\colon A\cap Y\to\mathbb{R}$ to a continuous convex function $F\colon A\to\mathbb{R}$. We characterize such extendability: (a) for a given $f$; (b) for every $f$. The case (b) for $A=X$ generalizes results from a paper by J. Borwein, V. Montesinos and J. Vanderwerff [Boundedness, differentiability and extensions of convex functions, J. Convex Analysis 13 (2006) 587--602], and from another one by L. Zaj\'{\i}\v{c}ek and the second author [On extensions of d.c.\ functions and convex functions, J. Convex Analysis 17 (2010) 427--440]. We also show that if $X$ is locally convex and $X/Y$ is ``conditionally separable'', then the couple $(X,Y)$ satisfies the $\mathrm{CE}$-property, saying that the above extendability holds for $A=X$ and every $f$. It follows that every couple $(X,Y)$ has the $\mathrm{CE}$-property for the weak topology. \par We consider also a stronger $\mathrm{SCE}$-property saying that the above extendability is true for every $A$ and every $f$. A deeper study of the $\mathrm{SCE}$-property will appear in a subsequent paper.

Keywords: Convex function, extension, topological vector space, normed linear space.

MSC: 52A41; 26B25, 46A99

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