作者机构:[1]Department of Mathematics, Harbin Institute of Technology, Heilongjiang 150001, P. R. China, [2]Department of Mathematics, Jilin University, Jilin 130012, P. R. China
相关基金:Supported by the National Natural Science Foundation of China (Grant No. 10371049) for the Doctoral Program of Higher Education of China (Grant No. 20050183002). Acknowledgements We are most indebted to our advisor, Professor JI Youqing, not only for his direct helpful suggestions but primarily for his continual instruction, encouragement and support over all these years.