1. School of Mathematical Sciences, Peking University, Beijing, 100871P.R. China.
2. Department of Mathematics, Pennsylvania State University, University Park, PA, 16802USA.
3. Department of Finance, The Smeal College of Business, Pennsylvania State University, University Park, PA, 16802USA.
4. Key Laboratory of Mathematical Economics and Quantitative Finance, Peking University, Beijing, 100871P.R. China.