1. Institut de Recherche Mathématique Avancée, UMR 7501, Université de Strasbourg et CNRS , 7 rue René-Descartes, 67000 Strasbourg CEDEX, France
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 and Mathematical Modeling , Almaty, 050010, Kazakhstan