Clearly,
+
+
6 − ( + + ). Therefore, as in [11] it can be shown that lim
−→∞
sup 1() 6 1
Thus, the considered region for model 3 is:
Γ = {( ) : + + 6 1 0 > 0 > 0}
Γ is positively invariant. i.e. every solution of model 3, with initial conditions in Γ remains there for all 0.