基金Acknowledgraent The first author is a master student, supervised hy Professor Chen between 2004-2005. The second author is an associate professor, and was a ph.d student supervised by Professor Chen between 1998-2002. The last author is an associate professor, and was a postdoe supervised by Professor Chen between 1999-2001. The three authors would like to attribute the accomplishment of this paper to the advice of their respectable director Professor Xiru Chen and express their gratitude and yearns to him. The authors are also grateful to Professor Zhongyi Zhu for his kind help. This project is partly supported by National Natural Science Foundation of China and President Foundation of GUCAS.