FONDA, A.; SABATINI, M.; ZANOLIN, F. Periodic solutions of perturbed Hamiltonian systems in the plane by the use of the Poincaré-Birkhoff theorem. Topological Methods in Nonlinear Analysis, [S. l.], v. 40, n. 1, p. 29–52, 2012. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2012.021. Acesso em: 22 jul. 2024.