1. Faculty of Mechanics and Mathematics, Taras Shevchenko National University of Kyiv, Volodymyrska 60, Kyiv 01033, Ukraine
2. Faculty of Applied Mathematics Silesian University of Technology, Kaszubska 23, Cliwice 44–100, Poland
3. Department of Mathematics, National University of Kyiv-Mohyla Academy, Skovorody 2, Kyiv 04070, Ukraine