1. National Institute for Mathematical and Biological Synthesis; University of Tennessee; Knoxville; 37996; TN; USA
2. Department of Mathematics; University of Tennessee; Knoxville; 37996; TN; USA
3. Department of Mathematics; University of Manitoba; Winnipeg; Manitoba; R3T 2N2; Canada
4. Department of Comparative Medicine; University of Tennessee; Knoxville; 37996; TN; USA