Li Zhen CHEN School of Mathematical Sciences,Xiamen University,Xiamen 361005,P.R.China Jian WANG School of Mathematics and Computer Science,Fujian Normal University, Fuzhou 350007,P.R.China
Acta Mathematica Sinica, Chinese Series
.
2009, (05): 191
-196
.
DOI: 10.12386/A2009sxxb0125
