1. Department of Mathematics Guizhou University Guiyang, Guizhou 550025, P.R. China
2. Department of Mathematical Analysis and Numerical Mathematics Comenius University in Bratislava Mlynská dolina, 842 48 Bratislava, SLOVAKIA and Mathematical Institute of Slovak Academy of Sciences Šanikova 49, 814 73 Bratislava, Slovakia
3. Department of Mathematics Xiangtan University Xiangtan, Hunan 411105, P.R. China