1. 1Institute of Mathematics, National Academy of Sciences of Belarus, 11 Surganov Str., 220072 Minsk, Belarus.
2. 2Department of Mathematics, The John Paul II Catholic University of Lublin, Al. Raclawickie 14, 20-950 Lublin, Poland.
3. 3Belarusian State University, 4, Nezavisimosti Ave., 220030, Minsk, Belarus.