1. Department of Mathematics , Guizhou University , Guiyang , Guizhou 550025 , China
2. Department of Mathematical Analysis and Numerical Mathematics, Faculty of Mathematics, Physics and Informatics , Comenius University in Bratislava, Mlynská dolina, 842 48 Bratislava, Slovakia, and Mathematical Institute, Slovak Academy of Sciences , Štefánikova 49, 814 73 Bratislava , Slovakia
3. School of Mathematics , Renmin University of China , Beijing 100872 , China