1. School of Economics and Management, Tsinghua University, Beijing 100084, China; 2. Institute of Systems Science, Academia Sinica, Beijing 100080, China
This paper is financial supported by a project of Financial Mathematics,Financial Engineering and Financial Management, which is one of "Ninth Five-Year Plan" Major Projects of National Natural Science Foundation of China (Grant 79790130) and Xiao Lin-Shi Foundation of Chinese Bconomic Research in School of Economics and Management, Tsinghua University.