FRIGON, M. Boundary and periodic value problems for systems of nonlinear second order differential equations. Topological Methods in Nonlinear Analysis, [S. l.], v. 1, n. 2, p. 259–274, 1993. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.1993.019. Acesso em: 27 apr. 2024.