1. Department of Mathematics, Duke University, Durham, North Carolina, U.S.A.
2. Department of Mathematics, Iowa State University, Ames, Iowa, U.S.A.
3. Department of Mathematics, Center for Theoretical and Mathematical Science, Center for Nonlinear and Complex Systems, and Department of Statistical Sciences, Duke University, Durham, North Carolina, U.S.A.
4. Department of Mathematics, Tulane University, New Orleans, Louisiana, U.S.A.
5. Department of Statistical Science, Department of Computer Science, Program in Computational Biology and Bioinformatics, Program in Structural Biology and Biophysics, Duke University, North Carolina, U.S.A.