Cheng, Sui Sun, and Yuan Huang Shao. “Comparison Theorems and Necessary Sufficient Conditions for Existence of Nonoscillatory Solutions of Forced Impulsive Delay Differential Equations”. Ukrains’kyi Matematychnyi Zhurnal, vol. 64, no. 9, Sept. 2012, pp. 1233-48, https://umj.imath.kiev.ua/index.php/umj/article/view/2654.