1. Department of Mathematics, Suleyman Demirel University, Almaty 050040, Kazakhstan
2. Department of Mathematics, Wayne State University, Detroit, MI 48202, USA
3. Department of Mathematics, Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan
4. Institute of Mathematics of the SB of RAS, Novosibirsk 630090, Russia
5. Institute of Mathematics and Mathematical Modeling, Almaty 050010, Kazakhstan