Complex behavior of COVID-19’s mathematical model Zhen Wang, Sajjad Shaukat Jamal, Baonan Yang and Viet-Thanh Pham Eur. Phys. J. Spec. Top., 231 5 (2022) 885-891 Published online: 17 November 2021 DOI: 10.1140/epjs/s11734-021-00309-4