Akkaoui, Ahmed, et al. “A Variant of d’Alembert’s Functional Equation on Semigroups With Endomorphisms”. Annales Mathematicae Silesianae, vol. 36, no. 1, Mar. 2022, pp. 1-14, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13456.