1. Department of Mathematics and Statistics, Texas Tech University, Lubbock, TX 79409-1042 USA.
2. Department of Mathematics and Statistics, Texas Tech University, Lubbock, TX 79409-1042 USA; Center for Applied Mathematics, Cornell University, Ithaca, NY 14853-6201 USA.
3. Corresponding author. Department of Mathematics and Statistics, Texas Tech University, Lubbock, TX 79409-1042 USA.
4. School of Computing and Mathematical Sciences, University of Leicester, Leicester LE1 7RH UK; Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow 119071 Russia.
5. Department of Physics and Astronomy, Texas Tech University, Lubbock, TX 79409-1051 USA.