RomanyukA. S., and SamoilenkoA. M. “Bogolyubov Readings-2008. International Conference "Differential Equations, Theory of Functions and Applications" (on the Occasion of the 70th Anniversary of Academician AM Samoilenko)”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 60, no. 12, Dec. 2008, p. 1722, https://umj.imath.kiev.ua/index.php/umj/article/view/3285.