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On quadratic set valued functions. (English) Zbl 0537.39002
A set valued function $$U:{\mathbb{R}}\to 2^ X$$ (where X is a real normed space) is said to be quadratic iff $$U(s+t)+U(s-t)=2U(s)+2U(t),$$ for all s,$$t\in {\mathbb{R}}$$. There is proved, among others, that if a quadratic set valued function U:$${\mathbb{R}}\to CC(X)$$ (where CC(X) denotes the family of all compact, convex and non-empty subsets of X) is bounded on a subset of $${\mathbb{R}}$$ of positive inner Lebesgue measure or if it is measurable, then it is of the form $$U(t)=t^ 2U(1),$$ $$t\in {\mathbb{R}}$$.

##### MSC:
 39B99 Functional equations and inequalities 39B52 Functional equations for functions with more general domains and/or ranges