1. Department of Mathematics, Stanford University, Stanford, CA 94305 USA.
2. Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 USA.
3. Amazon Search, Palo Alto, CA 94303 USA.
4. Department of Mathematics, Stanford University, Stanford, CA 94305-2125 USA, and Amazon Search, Palo Alto, CA 94303 USA.
5. Department of Computer Sciences, University of Texas at Austin, Austin, TX 78712-1188 USA, and Google.