= Solution
For $\mu\gg1$, trajectories rapidly approach the cubic nullcline
$$
y=\mu(x^3/3-x)
$$
and then move slowly along its attracting outer branches $|x|>1$. For $0<F<1$, the equilibrium lies on the repelling middle branch. Slow motion to a fold, a rapid jump to the opposite outer branch, and repetition produce a <relaxation oscillation>. For $F>1$, the equilibrium lies on the attracting right branch; trajectories undergo any necessary fast jump and then drift to that equilibrium, so no relaxation cycle remains.
Back to article page
