Abakumov, S., Bohdan Khmelnytsky National University of Cherkasy, 81 Shevchenko Blvd., UA-18031 Cherkasy, Ukraine Andreieva, Yu. A., Vasyl’ Stus Donetsk National University, Vinnytsia, Ukraine Atamas’, I. V. Atamas’, V. V., The Bohdan Khmelnytsky National University of Cherkasy
