Hong Ya GAO College of Mathematics and Computer Science,Hebei University,Baoding 071002,P.R.China Hebei Provincial Center of Mathematics,Shijiazhuang 050016,P.R.China Li Fang ZHAO College of Mathematics and Computer Science,Hebei University,Baoding 071002
Acta Mathematica Sinica, Chinese Series
.
2009, (02): 47
-52
.
DOI: 10.12386/A2009sxxb0030
