Duc, Duong Minh, Tran Vinh Hung, and Nguyen Tien Khai. “Critical Points of Non-$C^{2}$ Functionals”. Topological Methods in Nonlinear Analysis 29, no. 1 (March 1, 2007): 35–68. Accessed March 29, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2007.003.