LIN, X.; JIANG, D.; O’REGAN, D.; AGARWAL, R. P. Twin positive periodic solutions of second order singular differential systems. Topological Methods in Nonlinear Analysis, [S. l.], v. 25, n. 2, p. 263–273, 2005. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2005.013. Acesso em: 25 apr. 2024.