1. Department of Basic Courses, Nanjing Institute of Technology, Nanjing 211167, China
2. Faculty of Science, Jiangsu University, Zhenjiang, Jiangsu 212013, China
3. 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
4. Department of Mathematics, University of Caldas, Manizales 275, Colombia
5. Department of Mathematics, University of Central Florida, Orlando, FL 32816-1364, USA