Lupo, Daniela. “Existence and Multiplicity of Solutions for Superquadratic Noncooperative Variational Elliptic Systems”. Topological Methods in Nonlinear Analysis 12, no. 1 (September 1, 1998): 27–46. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.1998.026.