1. School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou 730070, China
2. School of Mathematics and Information Science, North Minzu University, Yinchuan 750021, China
3. School of Mathematics, Southwest Jiaotong University, Chengdu 611756, China
4. Department of Electrical Engineering, Polytechnique Montréal, P. O. Box 6079, Station Centre-Ville, Montreal, QC, Canada