Kolomiyets, V.G., Samoilenko, A.M. and Samoilenko, V.G. 1998. The Third Bogolyubov Readings. International Scientific Conference “Asymptotic and Qualitative Methods in the Theory of Nonlinear Oscillations”. Ukrains’kyi Matematychnyi Zhurnal. 50, 4 (Apr. 1998), 599.