Precup, Radu, and Csaba Varga. “Localization of Positive Critical Points in Banach Spaces and Applications”. Topological Methods in Nonlinear Analysis 49, no. 2 (May 22, 2017): 817–833. Accessed April 18, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2017.011.