School of Math. and Statist.;
Hebei Univ. of Economics and Business;
Shijiazhuang 050061;
China;
Dept. of Math.;
Tianjin Polytechnic Univ.;
Tianjin 300160;
China;
Institute of Appl. Math. and Mech.;
Ordnance Engineer. College;
Shijiazhuang 050003;
China;