Medak, B., A. A. Tret’yakov, and H. Żołądek. “Solutions to Some Singular Nonlinear Boundary Value Problems”. Topological Methods in Nonlinear Analysis, vol. 41, no. 2, Apr. 2013, pp. 255-6, https://apcz.umk.pl/TMNA/article/view/TMNA.2013.011.