Hakl, Robert, Pedro J. Torres, and Manuel Zamora. “Periodic Solutions to Singular Second Order Differential Equations: The Repulsive Case”. Topological Methods in Nonlinear Analysis 39, no. 2 (April 23, 2012): 199–220. Accessed April 25, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2012.012.