1. School of Statistics; Southwestern University of Finance and Economics; Chengdu Sichuan 611130 China
2. School of Mathematics and Statistics; University of Melbourne; Parkville Victoria 3010 Australia
3. Department of Statistics; Colorado State University; Fort Collins Colorado 80523 U.S.A.
4. Department of Operations Research and Financial Engineering; Princeton University; Princeton New Jersey 08544 U.S.A.
5. School of Statistics; University of Minnesota; Minneapolis Minnesota 55455 U.S.A.