1. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, China
2. Center for Applied Mathematics and Theoretical Physics, University of Maribor, Krekova 2, 2000 Maribor, Slovenia
3. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
4. School of Basic Sciences, Indian Institute of Technology Mandi, Mandi 175001, India