O’REGAN, Donal. Upper and lower solutions for problems with singular sign changing nonlinearities and with nonlinear boundary data. Topological Methods in Nonlinear Analysis, [S. l.], v. 19, n. 2, p. 375–390, 2002. Disponível em: https://apcz.umk.pl/TMNA/article/view/TMNA.2002.019. Acesso em: 2 jan. 2025.