1. Department of Applied Mathematics, University of Science and Technology Beijing, Beijing 100083, China
2. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620, USA
3. International Institute for Symmetry Analysis and Mathematical Modelling, Department of Mathematical Sciences, North-West University, Mafikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa
4. Department of Mathematics, Beijing University of Chemical Technology, Beijing 100029, China