Ben-Naoum, A. K. and Mawhin, J. (1993) “Periodic solutions of some semilinear wave equations on balls and on spaces”, Topological Methods in Nonlinear Analysis, 1(1), pp. 113–137. Available at: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.010 (Accessed: 22 October 2021).