1. School of Mathematics and Computer Science, Ningxia University, Yinchuan 750021, P. R. China
2. Department of Mathematics, Tongji University, Shanghai 200092, P. R. China
3. Department of Mathematics, Hefei University of Technology, Hefei 230009, P. R. China
4. Department of Mathematics, China University of Mining and Technology, Xuzhou 221116, P. R. China