1. School of Mathematics and Statistics, Nanjing University of Information Science and Technology, Nanjing 210044, P. R. China
2. Department of Mathematics, Shanghai Normal University, Shanghai 200234, P. R. China
3. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao 266590, P. R. China
4. Center for Applied Mathematics and Theoretical Physics and Faculty of Natural Science and Mathematics, University of Maribor, Maribor, SI 2000, Slovenia