1. School of Economic Mathematics, Southwestern University of Finance and Economics, Chengdu 611130, People's Republic of China
2. Guiyang Institute for Big Data and Finance, Guizhou University of Finance and Economics, Guiyang 550025, People's Republic of China
3. Current address: Department of Statistics and Actuarial Science, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.