1. School of Mathematical Sciences, Dalian University of Technology, Dalian, Liaoning 116024, People’s Republic of China.
2. Department of Mathematics and Statistics, Georgia State University, Atlanta, GA 30303 USA.
3. Mathematics Department, Louisiana State University, Baton Rouge, LA 70803-4918 USA.
4. Corresponding author. School of Mathematical and Data Sciences, West Virginia University, Morgantown, WV 26506 USA.
5. Department of Mathematics, University of Hong Kong, Hong Kong, People’s Republic of China.