Habets, P., and M. Gaudenzi. “Existence and Multiplicity of Positive Solutions for Boundary Value Problems of 2d Order ODE”. Topological Methods in Nonlinear Analysis, vol. 14, no. 1, Sept. 1999, pp. 131-50, https://apcz.umk.pl/TMNA/article/view/TMNA.1999.024.