Cheng, S.S. and 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), pp. 1233–1248. Available at: https://umj.imath.kiev.ua/index.php/umj/article/view/2654 (Accessed: 6 December 2025).