, Vasyl’ Stus Donetsk National University, Vinnytsia, Ukraine (Ukraine) , Vasyl’ Stus Donetsk National University, Vinnytsia, Ukraine , Institute of Mathematics, Cracow University of Technology, Krakow, Poland (Poland) , Department of Mathematics, Kamianets-Podilskyi Ivan Ohienko National University,
Kamianets-Podilskyi, Ukraine (Ukraine)
