Lakusta, L. M., R. I. Petryshyn, and A. M. Samoilenko. 2002. “Averaging of Boundary-Value Problems With Parameters for Multifrequency Impulsive Systems”. Ukrains’kyi Matematychnyi Zhurnal 54 (9), 1237-49. https://umj.imath.kiev.ua/index.php/umj/article/view/4162.