Prástaro, A. and Rassias, T. M. (2001) “Results on the J. d’Alembert equation”, Annales Universitatis Paedagogicae Cracoviensis Studia Mathematica, 1, pp. 117–128. Available at: https://studmath.uken.krakow.pl/article/view/13522 (Accessed: 30 October 2026).