1. Department of Applied Mathematics, School of Mathematics and Physics, University of Science and Technology, Beijing 100083, China
2. Department of Mathematics, University of Texas at San Antonio, San Antonio, TX 78249, USA School of Mathematics and Statistics, The Central South University, Changsha 410083, China
3. Division of Mathematics, University of Dundee, Dundee, DD1 4HN, UK
4. Center for Applied Mathematics, Tianjin University, Tianjin, 300072, China