1. Department of Mathematics, Shanghai Normal University, Shanghai, People's Republic of China
2. Department of Epidemiology and Health Statistics, Fudan University, Shanghai, People's Republic of China
3. Department of Biology, Case Western Reserve University, Cleveland, OH, USA
4. Department of Mathematical and Computational Sciences, The College of Wooster, Wooster, OH, USA
5. Department of Mathematics and Statistics, Cleveland State University, Cleveland, OH, USA