On functional equations associated with characters of unitary representations of groups.
Christensen measurable solutions of generalized Cauchy functional equations.
On functional polynomials spanned by characters of unitary representations. (Summary).
A solution to a problem of J. Schwaiger.
On some properties of Hamel bases connected with the continuity of polynomial functions.
Additive and convex functions in linear topological space.
On trigonometric polynomials spanned by characters of unitary representations.
On stability of the Cauchy equation on semigroups.
Christensen measurable solutions of generalized Cauchy functional equations. (Short Communications).
Weak difference properties of higher orders for the class
Christensen measurability of polynomial functions and convex functions of higher orders
Difference properties of higher orders for continuity and Riemann integrability
Local stability of the functional equation characterizing polynomial functions
On separation theorems for subadditive and superadditive functionals
We generalize the well known separation theorems for subadditive and superadditive functionals to some classes of not necessarily Abelian semigroups. We also consider the problem of supporting subadditive functionals by additive ones in the not necessarily commutative case. Our results are motivated by similar extensions of the Hyers stability theorem for the Cauchy functional equation. In this context the so-called weakly commutative and amenable semigroups appear naturally. The relations between...
A theorem of the Hahn-Banach type and its applications
Let Y be a subgroup of an abelian group X and let T be a given collection of subsets of a linear space E over the rationals. Moreover, suppose that F is a subadditive set-valued function defined on X with values in T. We establish some conditions under which every additive selection of the restriction of F to Y can be extended to an additive selection of F. We also present some applications of results of this type to the stability of functional equations.
On stability of additive mappings.
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