Sfecci, Andrea. “Double Resonance in Sturm-Liouville Planar Boundary Value Problems”. Topological Methods in Nonlinear Analysis 55, no. 2 (May 30, 2020): 655–679. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2019.109.