1. School of Mathematics Tianjin University Tianjin 300075 China
2. Department of Psychology and Neuroscience University of North Carolina at Chapel Hill Chapel Hill NC 27599‐3270 U.S.A
3. Department of Sociology University of North Carolina at Chapel Hill Chapel Hill NC 27599‐3270 U.S.A
4. Sprott School of Business Carleton University Ottawa Ontario K1S 5B6 Canada
5. Department of Statistics University of Manitoba Winnipeg Manitoba R3T 2N2 Canada