On the stability of the quadratic functional equation in topological spaces.
On the stability of the quadratic mapping in normed spaces.
On the stability of trigonometric functional equations.
On the structure of homoethic functions. (Summary).
On the structure of homothetic functions.
On the superstability of generalized d’Alembert harmonic functions
The aim of this paper is to study the superstability problem of the d’Alembert type functional equation f(x+y+z)+f(x+y+σ(z))+f(x+σ(y)+z)+f(σ(x)+y+z)=4f(x)f(y)f(z) for all x, y, z ∈ G, where G is an abelian group and σ : G → G is an endomorphism such that σ(σ(x)) = x for an unknown function f from G into ℂ or into a commutative semisimple Banach algebra.
On the uniqueness of continuous solutions of a functional equation of n-th order.
On trigonometric functional equations of rectangular type.
On trigonometric polynomials spanned by characters of unitary representations.
On two conditional forms of the equation ||f(x + y)|| = ||f(x) + f(y)||.
On two conditional forms of the equation ||f(x + y)|| = ||f(x) + f(y)||. (Summary).
On two mean value properties and functional equations associated with them.
Opérateurs multiplicativement liés
Orbit properties of functions and "Pre-Abel'' equations
Orthogonal and additivity modulo a subgroup.
Orthogonality and additivity modulo a subgroup. (Summary).