Lehlou, F., Moussa, M., Roukbi, A., & Kabbaj, S. (2016). On the superstability of the cosine and sine type functional equations. Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, 15, 113–121. Retrieved from https://studmath.uken.krakow.pl/article/view/13147