1. School of Economics and Statistics, Guangzhou University, 230 Wai Huan Xi Road, Guangzhou 510006, China
2. Department of Statistics, Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A
3. Department of Statistics, School of Economics, Jinan University, 601 Huangpu W. Ave., Guangzhou 510632, China
4. Department of Statistics and Applied Probability, National University of Singapore, 21 Lower Kent Ridge Road, 117546 Singapore