作者机构:[1]Department of Mathematics, Zhengzhou University, Zhengzhou 450052, China, [2]School of Mathematical Sciences, Xuzhou Normal University, Xuzhou 221116, China
相关基金:Supported by the National Natural Science Foundation of China under Grant No. 10871165 .Acknowledgments The authors would like to thank Prof. Ce-Wen Cao for his enthusiastic guidance and helpful discussions