Xu, J., Z. Wei, and Y. Ding. “Existence of Positive Solutions for a Second Order Periodic Boundary Value Problem With Impulsive Effects”. Topological Methods in Nonlinear Analysis, vol. 43, no. 1, Apr. 2016, pp. 11-22, https://apcz.umk.pl/TMNA/article/view/TMNA.2014.002.