Massa, Eugenio, and Bernhard Ruf. “On the Fučík Spectrum for Elliptic Systems”. Topological Methods in Nonlinear Analysis 27, no. 2 (June 1, 2006): 195–228. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2006.008.