An early version of the paper was presented at the 9th International Conference on Computing in Economics and Finance and the First Cross-Strait Conference on Statistical Physics. It was also presented as a part of a sequence of intensive lectures given at the University of Science and Technology of China (USTC). The authors are grateful to Prof. Bing-Hong Wang for hosting the authors' visit at USTC. The authors would also like to thank the two anonymous referees for their helpful suggestions. NSC research grant No. 92-2415-H-004-005 is gratefully acknowledged.
International Journal of Modern Physics B 18(17n19), pp.2376-2386