1. Institute of Mathematics, Physics and Informatics, Abai Kazakh National Pedagogical University, 86 Tole Bi Street, Almaty 050012, Kazakhstan
2. Department of Higher Mathematics, National Research University “TIIAME”, 39 Kari-Niyazi Street, Tashkent 100000, Uzbekistan
3. Department of Mathematics, Analysis, Logic and Discrete Mathematics, Ghent University, 9000 Gent, Belgium