Papini, Duccio, and Fabio Zanolin. “A Topological Approach to Superlinear Indefinite Boundary Value Problems”. Topological Methods in Nonlinear Analysis 15, no. 2 (June 1, 2000): 203–233. Accessed May 4, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2000.017.