Xu, Jiafa, Zhongli Wei, and Youzheng Ding. “Existence of Positive Solutions for a Second Order Periodic Boundary Value Problem With Impulsive Effects”. Topological Methods in Nonlinear Analysis 43, no. 1 (April 12, 2016): 11–22. Accessed July 1, 2025. https://apcz.umk.pl/TMNA/article/view/TMNA.2014.002.