LI, Shujie; SZULKIN, Andrzej. Periodic solutions of an asymptotically linear wave equation. Topological Methods in Nonlinear Analysis, [S. l.], v. 1, n. 2, p. 211–230, 1993. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.016. Acesso em: 8 jul. 2025.