1. Department of Mathematics, Texas A&M University-Commerce, Monroe St , Commerce , 75402, TX , USA
2. Department of Mathematics, University of Pittsburgh, Fifth Avenue , Pittsburgh , 15260, Pennsylvania , USA
3. The Intercollegiate Biomathematics Alliance, Illinois State University , Normal , 61790, Illinois , USA
4. Department of Mathematical Sciences, George Mason University, 4400 University Drive , Fairfax , 22030, Virginia , USA