1. 4 Department of Mathematics , Guizhou University , Guiyang, Guizhou 550025, P.R. 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 of Slovak Academy of Sciences Štefánikova 49, 814 73 Bratislava , Slovakia
3. Department of Mathematics , Guizhou University Guiyang , Guizhou 550025, P.R. China e-mail: wjr9668@126.com