1. Mathematics Department, Swarthmore College , 500 College Avenue , Swarthmore PA 19081 , USA
2. Department of Applied and Computational Mathematics and Statistics , University of Notre Dame , 153 Hurley Hall , Notre Dame , IN 46556 , USA
3. Department of Mathematics , University of Maryland , 4176 Campus Drive, College Park, MD 20742 , Maryland , USA
4. Department of Mathematics and Department of Computer Science , Brown University , 151 Thayer Street , Providence , RI 02912 , USA