\frac{dx_{1}}{dt} = k_{3} \cdot \left(x_{1} + \left(-x_{1}^{3} / 3\right) + x_{2} + k_{4}\right)\\
\frac{dx_{2}}{dt} = \left(-1 / k_{3}\right) \cdot \left(x_{1} + \left(-k_{1}\right) + k_{2} \cdot x_{2}\right)