Averaged first-integral obstruction to persistence of a periodic orbit (source code)

= Averaged first-integral obstruction to persistence of a periodic orbit

Suppose $V$ is a <first integral> of an unperturbed planar system and $\dot V=\varepsilon G+O(\varepsilon^2)$ after perturbation. A periodic orbit converging to an unperturbed orbit $\gamma$ can persist only if
$$
\int_\gamma G\,dt=0.
$$
Indeed, the net change of $V$ over every period is zero. A nonzero averaged drift therefore obstructs persistence.