1. Department of Mathematics, M. Auezov South Kazakhstan State University, Tauke Han av., 5, 160012 Shymkent, Kazakhstan
2. Department of Mathematics, Fatih University, Buyukcekmece, 34500 Istanbul, Turkey
3. Department of Mechanics and Mathematics, N.G. Chernyshevsky Saratov State University, Astrakhanskaya, 83, 410012 Saratov, Russia