CINGOLANI, S.; LAZZO, M. Multiple periodic solutions for autonomous conservative systems. Topological Methods in Nonlinear Analysis, [S. l.], v. 14, n. 1, p. 159–167, 1999. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1999.026. Acesso em: 22 jul. 2024.