1. Department of Mathematics, University of Arizona, Tucson, AZ 85721 USA.
2. Interdisciplinary Program in Applied Mathematics, University of Arizona, Tucson, AZ 85721 USA.
3. Department of Mathematics, Interdisciplinary Program in Applied Mathematics, Department of Epidemiology and Biostatistics, BIO5 Institute, University of Arizona, Tucson, AZ 85721 USA.
4. Department of Mathematics, Department of Epidemiology and Biostatistics, BIO5 Institute, University of Arizona, Tucson, AZ 85721 USA.