Cheng, S. S., & Shao, Y. H. (2012). Comparison theorems and necessary/sufficient conditions for existence of nonoscillatory solutions of forced impulsive delay differential equations. Ukrains’kyi Matematychnyi Zhurnal, 64(9), 1233-1248. Retrieved from https://umj.imath.kiev.ua/index.php/umj/article/view/2654