Jin Su HE College of Mathematics,Physics and Information Science,Zhejiang Normal University, Jinhua 321004,P.R.China Xian Fa LUO College of Sciences,China Jiliang University,Hangzhou 310018,P.R.China
Acta Mathematica Sinica, Chinese Series
.
2009, (03): 201
-210
.
DOI: 10.12386/A2009sxxb0075
