Li, Yi, Zhaoli Liu, and Cunshan Zhao. “Nodal Solutions of Perturbed Elliptic Problem”. Topological Methods in Nonlinear Analysis 32, no. 1 (September 1, 2008): 49–68. Accessed April 25, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2008.035.