405258.fig.002a
(a)
405258.fig.002b
(b)
405258.fig.002c
(c)
Figure 2: Backward bifurcation regions for the model (4) in the - parameter space corresponding to and various ranges of . Parameter values used are: ,   . In (a) (backward bifurcation region for is , (b) (backward bifurcation region for is , and (c) (backward bifurcation region for is . With the above set of parameter values, , and .