1. Booth School of Business, University of Chicago, Chicago, Illinois 60637
2. School of Mathematical Sciences, University of Science and Technology of China, Hefei, Anhui Province, 230026, China; and Wu Wen Tsun Key Laboratory of Mathematics, Chinese Academy of Science, Hefei, Anhui Province 230026, China
3. Department of Industrial Engineering and Operations Research, Columbia University, New York, New York 10027