A new type of functional equations on semigroups with involutions

Iz-iddine El-Fassi

Published 2021-02-02Version 1

Let $S$ be a commutative semigroup, $K$ a quadratically closed commutative field of characteristic different from $2$, $G$ a $2$-cancellative abelian group and $H$ an abelian group uniquely divisible by $2$. The aim of this paper is to determine the general solution $f:S^2\to K$ of the d'Alembert type equation: $$ f(x+y,z+w)+f(x+\sigma(y),z+\tau(w)) =2f(x,z)f(y,w),\quad\quad (x,y,z,w\in S) $$ the general solution $f:S^2\to G$ of the Jensen type equation: $$ f(x+y,z+w)+f(x+\sigma(y),z+\tau(w)) =2f(x,z),\quad\quad (x,y,z,w\in S) $$ the general solution $f:S^2\to H$ of the quadratic type equation quation: $$ f(x+y,z+w)+f(x+\sigma(y),z+\tau(w)) =2f(x,z)+2f(y,w),\quad\quad (x,y,z,w\in S) $$ where $\sigma,\tau: S\to S$ are two involutions.