1. School of Mathematics; Southwest Jiaotong University; Chengdu 610031 PR China
2. Department of Mathematics; Qingdao Technological University; Qingdao 266520 PR China
3. Department of Mathematics; Harbin Institute of Technology; Harbin 150001 PR China
4. Department of Mathematical and Statistical Sciences; University of Alberta; Edmonton AB, T6G 2G1 Canada