Grossi, Massimo, Filomena Pacella, and S. L. Yadava. “Symmetry Results for Perturbed Problems and Related Questions”. Topological Methods in Nonlinear Analysis 21, no. 2 (June 1, 2003): 211–226. Accessed December 8, 2021. https://apcz.umk.pl/TMNA/article/view/TMNA.2003.013.