Drábek, P., and M. Langerová. “On the Second Order Equations With Nonlinear Impulses - Fredholm Alternative Type Results”. Topological Methods in Nonlinear Analysis, vol. 44, no. 1, Apr. 2016, pp. 249-61, https://apcz.umk.pl/TMNA/article/view/TMNA.2014.046.