Hao LIU Institute of Mathematics,College of Mathematics and Information Science, Henan University,Kaifeng 475001,P.R.China Jie HONG Department of Mathematics,Shaanxi University of Technology,Hanzhong 723001,P.R.China
Acta Mathematica Sinica, Chinese Series
.
2009, (04): 89
-98
.
DOI: 10.12386/A2009sxxb0087
