1. Department of Mathematics, The John Paul II Catholic University of Lublin, Al. Raclawickie 14, Lublin, 20–950, Poland
2. Department of Mathematics, The John Paul II Catholic University of Lublin, Al. Raclawickie 14, Lublin, 20–950, Poland ; Institute of Mathematics, NAS of Belarus, Surganov St.11, Minsk, 220072, Belarus