1. Department of Mathematics and Statistics; University of South Florida; Tampa FL 33620-5700 USA
2. College of Mathematics and Systems Science; Shandong University of Science and Technology; Qingdao 266590 Shandong PR China
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