1. College of Mathematical Sciences, Yangzhou University, Yangzhou 225002, P. R. China
2. School of Mathematics and Statistics, Hubei Normal University, Huangshi 435002, P. R. China
3. Faculty of Sciences and Mathematics, University of Niš, P. O. Box 224, 18000 Niš, Serbia
4. School of Mathematics, Southeast University, Nanjing 210096, P. R. China