Stability, Bifurcation, and a Pair of Conserved Quantities in a Simple Epidemic System with Reinfection for the Spread of Diseases Caused by Coronaviruses

Figure 2

Representation of nullclines of system (9): horizontal direction (in blue) and vertical direction (in red); the vertical axis is a combination of both. Equilibrium points are indicated by black circles. (a) , and ; (b) , and .

(a)

(b)