BEN-NAOUM, A. K.; MAWHIN, J. Periodic solutions of some semilinear wave equations on balls and on spaces. Topological Methods in Nonlinear Analysis, [S. l.], v. 1, n. 1, p. 113–137, 1993. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.010. Acesso em: 22 oct. 2021.