1. CNRS, IRMAR-UMR 6625, Univ. Rennes, F-35000, Rennes, France
2. School of Mathematical Sciences, Fudan University, Shanghai 200433, China
3. Zhongtai Securities Institute for Financial Studies, Shandong University, Jinan 250100, China
4. School of Mathematics, Shandong University, Jinan 250100, China