Figure 4.

Bifurcation diagrams versus parameter r for illustrating the two and three coexisting attractors of the system (2): (a) $\sigma =2$, $\delta =1.5$, $b=0.7$ for the initial values $(1,1,1,1)$ (red) and $(-2,1,1,1)$ (blue); (b) $\sigma =4$, $\delta =0.5$, $b=2$ for the initial values $(2,1,1,2)$ (blue), $(-2,1,1,-2)$ (red) and $(2,1,1,-2)$ (green).