Figure 3.

The chaotic attractor obtained with $\left(a_1,a_2,a_3,a_4\right)=\left(0.51,1,1.51,0.74\right)$ and $\left(x\left(0\right),y\left(0\right)\right)=\left(-0.81,0.51\right)$ for different fractional orders $\upsilon$.