FONDA, A.; GHIRARDELLI, L. Multiple periodic solutions of Hamiltonian systems in the plane. Topological Methods in Nonlinear Analysis, [S. l.], v. 36, n. 1, p. 27–38, 2010. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2010.026. Acesso em: 27 apr. 2024.