Zu, Li, Xiaoning Lin, and Daqing Jiang. “Existence Theory for Single and Multiple Solutions to Singular Boundary Value Problems for Second Order Impulsive Differential Equations”. Topological Methods in Nonlinear Analysis 30, no. 1 (September 1, 2007): 171–191. Accessed April 26, 2024. https://apcz.umk.pl/TMNA/article/view/TMNA.2007.024.