1. School of Mathematics, Hefei University of Technology, Hefei 230009, China
2. School of Mathematics, Hefei University of Technology, Hefei 230009, China.
3. CMAT-Centro de Matemática, Universidade do Minho, Braga 4710-057, Portugal.
4. Faculty of Sciences and Mathematics, University of Niš, P.O. Box 224, 18000 Niš, Serbia.