Lyapunov function for $\left(\begin{array}{c}x'\\y'\end{array}\right)=\left(\begin{array}{c}x-y-x^3\\x+y-2y^3\end{array}\right)$

Question

I am looking for a Lyapunov function for the Nonlinear system $$\left(\begin{array}{c}x'\\y'\end{array}\right)=\left(\begin{array}{c}x-y-x^3\\x+y-2y^3\end{array}\right)$$

I am trying with $$V(x,y)=ax^n+by^m$$ But it doesn't work

Can you help me please?