1. Department of Basic Course, Southwest Jiaotong University, Emeishan, Sichuan614202, P. R. China; and School of Mathematics, Southwest Jiaotong University, Chengdu, Sichuan 611756, P. R. China
2. College of Mathematics and Statistics, Northwest Normal University, Lanzhou, Gansu730070, P. R. China
3. School of Mathematics and Statistics, Lanzhou University, Lanzhou, Gansu730000, P. R. China