A d'Alembert type functional equation on semigroups

Abstract: We treat two related trigonometric functional equations on semigroups. First we solve the $\mu$-sine subtraction law [\mu(y) k(x \sigma(y))=k(x) l(y)-k(y) l(x), \quad x, y \in S,] for $k, l : S\rightarrow \mathbb{C}$, where $S$ is a semigroup and $\sigma$ an involutive automorphism, $\mu :S\rightarrow \mathbb{C}$ is a multiplicative function such that $\mu (x\sigma (x))=1$ for all $x\in S$, then we determine the complex-valued solutions of the following functional equation [f(xy) - \mu (y)f(\sigma (y)x) = g(x)h(y),\quad x,y\in S,] on a larger class of semigroups.