Special Cubic Perturbations of the Duffing Oscillator $$x''=x-x^3$$ Near the Eight-Loop

We find an upper bound for the number of limit cycles, bifurcating from eight-loop Duffing oscillator $$x''= x-x^{3}$$ under special cubic perturbation $$\begin{aligned} x''= x-x^{3}+\lambda _{1}y+\lambda _{2}x^{2}+\lambda _{3}xy+\lambda _{4}x^{2}y. \end{aligned}$$