1. Department of Mathematics, Temple University, Philadelphia, PA 19122 USA.
2. Departments of Pediatrics and Bioengineering, University of California, San Diego, La Jolla, CA 92093 USA.
3. Department of Mathematical Sciences, Montana State University, Bozeman, MT 59717-2400 USA.
4. Department of Biology, San Diego State University, San Diego, CA 92182-4614 USA.