Gui Qiao XU Department of Mathematics,Tianjin Normal University,Tianjin 300387,P.R.CHina Ying Fang DU College of Chemistry and Life Science,Tianjin Normal University,Tianjin 300387,P.R.China
Acta Mathematica Sinica, Chinese Series
.
2009, (03): 109
-120
.
DOI: 10.12386/A2009sxxb0064
