1. School of Mathematics and Statistics, Huazhong University of Science and Technology, Wuhan, China, 430074 (e-mail: li_yun@hust.edu.cn)
2. Department of Mathematics, Worcester Polytechnic Institute, Hopkinton, -MA, United States of America, 01748 (e-mail: qsong@wpi.edu)
3. School of Mathematics and Statistics, Huazhong University of Science and Technology, Wuhan, China, 430074 (e-mail: wufuke@mail.hust.edu.cn)
4. Dept. of Math, Wayne State University, Detroit, MI, United States of America, 48202 (e-mail: gyin@wayne.edu)