1. School of Mathematics, Southwest Jiaotong University, Chengdu, China
2. Department of Mathematics and Statistics and the Hubei Key Laboratory of Intelligent Wireless Communications, South-Central University for Nationalities, Wuhan, China
3. Department of Informatics, University of Bergen, Bergen, Norway
4. Department of Mathematics, The Hong Kong University of Science and Technology, Hong Kong
5. Hubei Key Laboratory of Mathematical Sciences, School of Mathematics and Statistics, Central China Normal University, Wuhan, China