作者机构:[1]Department of Applied Mathematics, Nanjing University of Finance & Economics, Jiangsu 210003, China
相关基金:Foundation item: the National Natural Science Foundation of China (No. 60473034); the Natural Science Foundation of Jiangsu Provincial Colleges and Universities (No. 07KJD110065).
Acknowledgment The author would like to thank Professor SHA Zhen of Zhejiang University for his help and encouragement.