KolomiyetsV. G., SamoilenkoA. M., and SamoilenkoV. G. “The Third Bogolyubov Readings. International Scientific Conference ‘Asymptotic and Qualitative Methods in the Theory of Nonlinear Oscillations’”. Ukrains’kyi Matematychnyi Zhurnal 50, no. 4 (April 25, 1998): 599. Accessed November 24, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/4931.