1. Department of Mathematics, Schreiner University, Kerrville, TX 78028, USA.
2. Department of Mathematics, University of Michigan, Ann Arbor, MI 48109, USA.
3. Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI 48109, USA.
4. Department of Anesthesiology, University of Michigan, Ann Arbor, MI 48109, USA.