Figure 7.

Bifurcation diagrams with $a_1$ as the critical parameter and $\left(a_2,a_3,a_4\right)=\left(1,1.6,0.72\right)$ and $\left(x\left(0\right),y\left(0\right)\right)=\left(-0.26,0.18\right)$ for different fractional orders $\upsilon$.