O’Regan, Donal. “Upper and Lower Solutions for Problems With Singular Sign Changing Nonlinearities and With Nonlinear Boundary Data”. Topological Methods in Nonlinear Analysis, vol. 19, no. 2, June 2002, pp. 375-90, https://apcz.umk.pl/TMNA/article/view/TMNA.2002.019.