Solutions and stability of variant of Van Vleck's and D'Alembert's functional equations

Abstract

In this paper. (1) We determine the complex-valued solutions of the following variant of Van Vleck's functional equation<br /> $$int_{S}f(sigma(y)xt)dmu(t)-int_{S}f(xyt)dmu(t) = 2f(x)f(y), ;x,yin S,$$ where $S$ is a semigroup, $sigma$ is an involutive morphism of $S$, and $mu$ is a complex measure that is linear combinations of Dirac measures $(delta_{z_{i}})_{iin I}$, such that for all $iin I$, $z_{i}$ is contained in the center of $S$. (2) We determine the complex-valued continuous solutions of the following variant of d'Alembert's functional equation<br /> $$int_{S}f(xty)dupsilon(t)+int_{S}f(sigma(y)tx)dupsilon(t) = 2f(x)f(y), ;x,yin S,$$ where $S$ is a topological semigroup, $sigma$ is a continuous involutive automorphism of $S$, and $upsilon$ is a complex measure with compact support and which is $sigma$-invariant. (3) We prove the superstability theorems of the first functional equation.