1. National Pedagogical Dragomanov University, Pyrogova st., 9, Kyiv 01601, Ukraine. E-mail: avklevtsovskiy@gmail.com
2. Department of Mathematical Physics, Taras Shevchenko National University of Kyiv, Volodymyrska st., 64/13, Kyiv 01601, Ukraine. E-mail: melnyk@imath.kiev.ua