1. The Center for Economic Research, Shandong University , Jinan 250100, China
2. Department of Economics, Finance and Legal Studies, University of Alabama , Box 870224 , Tuscaloosa, AL 35487, USA
3. Department of Mathematics and Statistics, University of North Carolina at Charlotte , 9201 University City Boulevard , Charlotte, NC 28223, USA
4. College of Finance and Statistics, Hunan University , Lushan Road , Hunan 410082, China