1. Department of Mathematics, The University of Tennessee, Knoxville, TN 37996, USA
2. Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, AL 35487, USA
3. Department of Biological Sciences, The University of Alabama, Tuscaloosa, AL 35487, USA
4. Department of Neurology, Oregon Health and Science University, Portland, OR 97239, USA
5. Department of Mathematics, The University of Alabama, Tuscaloosa, AL 35487, USA