Akkaoui, Ahmed, Mohamed El Fatini, and Brahim Fadli. 2022. “A Variant of d’Alembert’s Functional Equation on Semigroups With Endomorphisms”. Annales Mathematicae Silesianae 36 (1):1-14. https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13456.