PetryshynYa. R., and SamoilenkoA. M. “Averaging Method in Multipoint Problems of the Theory of Nonlinear Oscillations”. Ukrains’kyi Matematychnyi Zhurnal 48, no. 8 (August 25, 1996): 1096-1103. Accessed July 27, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/5253.