1. College of Mathematics and Systems Science, Shandong University of Science and Technology, Qingdao 266590, P. R. China
2. Department of Mathematics and Information Technology, The Education University of Hong Kong, New Territories, Hong Kong
3. International Institute for Symmetry Analysis and Mathematical Modelling, Department of Mathematical Sciences, North-West University, Mafikeng Campus, Mmabatho X2046, South Africa