Clapp, Mónica, and Andrzej Szulkin. “Solutions to Indefinite Weakly Coupled Cooperative Elliptic Systems”. Topological Methods in Nonlinear Analysis 59, no. 2A (September 12, 2021): 553–568. Accessed July 22, 2024. https://apcz.umk.pl/TMNA/article/view/35206.