作者机构:[1]College of Mathematics and Information Science, Northwest Normal University, Gansu 730070, P. R. China
相关基金:Supported by the National Natural Science Foundation of China (Grant No. 10771091) and the Scientific Research Project of Northwest Normal University (Grant No. NWNU-KJCXGC-03-61). Acknowledgement The authors are very grateful to Prof. Zhongfu ZHANG for his helpful comments and suggestions.