Musso, M., and D. Passaseo. “Positive Solutions of Nonlinear Elliptic Problems Approximating Degenerate Equations”. Topological Methods in Nonlinear Analysis, vol. 6, no. 2, Dec. 1995, pp. 371-97, https://apcz.umk.pl/TMNA/article/view/TMNA.1995.051.