Solutions and stability of variant of Wilson's functional equation

Abstract: In this paper we will investigate the solutions and stability of the generalized variant of Wilson's functional equation $$ (E):\;\;\;\; f(xy)+\chi(y)f(\sigma(y)x)=2f(x)g(y),\; x,y\in G,$$ where $G$ is a group, $\sigma$ is an involutive morphism of $G$ and $\chi$ is a character of $G$. (a) We solve $(E)$ when $\sigma$ is an involutive automorphism, and we obtain some properties about solutions of $(E)$ when $\sigma$ is an involutive anti-automorphism. (b) We obtain the Hyers Ulam stability of equation $(E)$. As an application, we prove the superstability of the functional equation $f(xy)+\chi(y)f(\sigma(y)x)=2f(x)f(y),\; x,y\in G.$