A nonlinear capacitor–resistor electrical circuit can be modeled using the differential equations

˙ x = y, ˙ y = −x + x3 − (a0 + x)y, where a0 is a nonzero constant and x(t) represents the current in the circuit at time t . Sketch phase portraits when a0 > 0 and a0 < 0 and give a physical interpretation of the results.