Fig. 3.

Plots of the regions of stability of the cell population models for no feedback (i), linear feedback (ii), and saturating feedback (iii). In the case of no feedback, there are only stable solutions on the line α = 0, β < 0; otherwise, there is either extinction or unbounded growth. In the case of the linear feedback, there is never unbounded growth in cell numbers, and there is a steady state if α > 0, and extinction if α < 0. For the saturating feedback, there is a strip of the parameter space that permits stable solutions, when 0 < α < k₀/m₀ and β < k₁/m₁, and outside this region there is either extinction or unbounded growth. The crosses correspond to the five states in the (α, β)-parameter space from Fig. 2.