Castro, Alfonso, David G. Costa, and Ratnasingham Shivaji. “Multiple Positive Solutions for a Class of Variational Systems”. Topological Methods in Nonlinear Analysis 50, no. 1 (June 25, 2017): 1–8. Accessed July 3, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2017.016.