BONHEURE, D.; DE COSTER, C. Forced singular oscillators and the method of lower and upper solutions. Topological Methods in Nonlinear Analysis, [S. l.], v. 22, n. 2, p. 297–317, 2003. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2003.041. Acesso em: 22 jul. 2024.