1. Department of Computer Science, University of North Carolina, P.O. Box 26170, Greensboro, NC 27402–6170, USA
2. Department of Computer Sciences, The University of Texas at Austin, 1 University Station C0500 Taylor Hall 2.124, Austin, TX 78712–0233, USA
3. Department of Mathematics, The University of Texas at Austin, 2515 Speedway Rm 8, Austin, TX 78712–0233, USA
4. Department of Mathematical Sciences, Rensselaer Polytechnic Institute, Amos Eaton 301, 110 8th Street, Troy, NY 12180, USA