1. Department of Applied Mathematics, University of Science and Technology Beijing, Beijing 100083, P. R. China
2. Department of Mathematics, Zhejiang Normal University, Jinhua 321004, Zhejiang, P. R. China
3. Department of Mathematics, King Abdulaziz University, Jeddah, Saudi Arabia
4. Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620, USA
5. Department of Mathematical Sciences, International Institute for Symmetry Analysis and Mathematical Modelling, North-West University, Mafikeng Campus, Private Bag X 2046, Mmabatho 2735, South Africa
6. Department of Mathematics, Beijing University of Chemical Technology, Beijing 100029, P. R. China