FONDA, A.; RAMOS, M.; WILLEM, M. Subharmonic solutions for second order differential equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 1, n. 1, p. 49–66, 1993. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.007. Acesso em: 25 apr. 2024.