1. Department of Mathematics, University of Dayton , 300 College Park, Dayton, OH 45469 , USA
2. Division of Biostatistics, College of Public Health, The Ohio State University , 281 W Lane Ave, Columbus, OH 43210 , USA
3. School of Mathematical Sciences, University of Nottingham , University Park, Nottingham NG7 2RD , UK
4. Division of Epidemiology, College of Public Health, The Ohio State University , 281 W Lane Ave, Columbus, OH 43210 , USA
5. Department of Mathematics, The Ohio State University , 281 W Lane Ave, Columbus, OH 43215 , USA