1. College of Science, Harbin Engineering University, Harbin 150001, People’s Republic of China
2. College of Automation, Harbin Engineering University, Harbin 150001, People’s Republic of China
3. The Institute of Mathematical Sciences, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong
4. Department of Mathematics, University of Texas, Arlington, Texas 76019, USA
5. Department of Mathematics, Cape Breton University, Sydney, Nova Scotia B1P 6L2, Canada