FRIGON, M.; O’REGAN, D. Some general existence principles for ordinary differential equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 2, n. 1, p. 35–54, 1993. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.029. Acesso em: 28 mar. 2024.