作者机构:[1]Department of Mathematics, University of Science and Technology of China, Hefei 230026, China, [2]Department of Mathematics, Huzhou Teacher's College, Huzhou 313000, China
相关基金:Foundation item: Supported by the NNSF of China(10571164); Supported by Specialized Research Fund for the Doctoral Program of Higher Education(SRFDP)(20050358052); Supported by the INSF of Zhejiang Province(Y606197) The authors thank the referee for his/her thorough reviewing with useful suggestions and comments made to the paper.