1. School of Mathematics, Southwest Jiaotong University, Chengdu, People's Republic of China
2. School of Mathematics Sciences, University of Electronic Science and Technology of China, Chengdu, People's Republic of China
3. College of Mathematic and Information Science, Shandong Technology and Business University, Yantai, People's Republic of China
4. School of Sciences, Tianjin Polytechnic University, Tianjin, People's Republic of China