FURI, M.; SPADINI, M. Multiplicity of forced oscillations for the spherical pendulum. Topological Methods in Nonlinear Analysis, [S. l.], v. 11, n. 1, p. 147–157, 1998. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1998.009. Acesso em: 26 apr. 2024.