Lakusta L. M., PetryshynR. I., and SamoilenkoA. M. “Averaging of Boundary-Value Problems With Parameters for Multifrequency Impulsive Systems”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 54, no. 9, Sept. 2002, pp. 1237-49, https://umj.imath.kiev.ua/index.php/umj/article/view/4162.