Bonanno, G., and S. A. Marano. “Positive Solutions of Elliptic Equations With Discontinuous Nonlinearities”. Topological Methods in Nonlinear Analysis, vol. 8, no. 2, Dec. 1996, pp. 263-7, https://apcz.umk.pl/TMNA/article/view/TMNA.1996.032.