DEGIOVANNI, M.; FANNIO, L. O. Multiple periodic solutions of autonomous semilinear wave equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 3, n. 2, p. 209–219, 1994. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1994.011. Acesso em: 19 apr. 2024.