1. Department of Mathematics, Columbus State University, Columbus, GA 31907 USA.
2. School of Computer Science and Technology (School of Artificial Intelligence), Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, PR China, and LAMPS and Center for Diseases Modeling CDM, Department of Mathematics and Statistics, York University, Toronto M3J 1P3, ON, Canada.
3. Département de Mathématiques et de Statistique and Centre de Recherches Mathématiques, Université de Montréal, Montréal H3C 3J7, QC, Canada.
4. LAMPS and Center for Diseases Modeling CDM, Department of Mathematics and Statistics, York University, Toronto M3J 1P3, ON, Canada.