Lin, Xiaoning, Daqing Jiang, Donal O’Regan, i Ravi P. Agarwal. „Twin Positive Periodic Solutions of Second Order Singular Differential Systems”. Topological Methods in Nonlinear Analysis 25, no. 2 (czerwiec 1, 2005): 263–273. Udostępniono lipiec 22, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2005.013.