MOTREANU, D.; MOTREANU, V. V.; PAPAGEORGIOU, N. S. Periodic solutions for nonautonomous systems with nonsmooth quadratic or superquadratic potential. Topological Methods in Nonlinear Analysis, [S. l.], v. 24, n. 2, p. 269–296, 2004. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2004.028. Acesso em: 19 apr. 2024.