School of Mathematics, University of the Witwatersrand, Johannesbury, South Africa;
Institute of Discrete Mathematics, Inner Mongolia University of Nationalities,Inner Mongolia 028005, P.R.China;
Institute of Discrete Mathematics, Inner Mongolia University of Nationalities,Inner Mongolia 028005, P.R.China;
Center for Discrete Mathematics, Fuzhou University, Fujian 350002, P.R.China;
