Kaplun, Yu. I., A. M. Samoilenko, and V. G. Samoilenko. 2002. “Singularly Perturbed Equations With Impulse Action”. Ukrains’kyi Matematychnyi Zhurnal 54 (8), 1089-9. https://umj.imath.kiev.ua/index.php/umj/article/view/4149.