ORPEL, A. The existence of minimizers of the action functional without convexity assumption. Topological Methods in Nonlinear Analysis, [S. l.], v. 20, n. 1, p. 179–193, 2002. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2002.032. Acesso em: 29 mar. 2024.