DEGIOVANNI, Marco; FANNIO, Laura Olian. 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: 4 jul. 2025.