1. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, P. R. China
2. Faculty of Natural Science and Mathematics, University of Maribor, SI-2000 Maribor, Slovenia
3. Institute of Mathematics, Physics and Mechanics, SI-1000 Ljubljana, Slovenia
4. School of Science, Guilin University of Aerospace Technology, Guilin 541004, P. R. China
5. Faculty of Electrical Engineering and Computer Science, University of Maribor, SI-2000 Maribor, Slovenia
6. Center for Applied Mathematics and Theoretical Physics, SI-2000 Maribor, Slovenia