1. School of Science, Huzhou University, Huzhou 313000, P. R. China
2. School of Physics and Mandelstam Institute for Theoretical Physics, University of the Witwatersrand, Wits 2050, South Africa
3. Center for Quantum Spacetime (CQUeST), Sogang University, Seoul 121-742, South Korea
4. Department of Mathematics, University of Ghana, LG 62, Legon, Accra, Ghana