Perera, Kanishka, and Martin Schechter. “Multiple Nontrivial Solutions of Elliptic Semilinear Equations”. Topological Methods in Nonlinear Analysis 16, no. 1 (September 1, 2000): 1–15. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2000.027.