Lin, Xiaoning, Daqing Jiang, Donal O’Regan, and Ravi P. Agarwal. “Twin Positive Periodic Solutions of Second Order Singular Differential Systems”. Topological Methods in Nonlinear Analysis 25, no. 2 (June 1, 2005): 263–273. Accessed July 6, 2025. https://apcz.umk.pl/TMNA/article/view/TMNA.2005.013.