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A nearly Hamiltonian system is a weak perturbation of a Hamiltonian system. Along its paths, $\dot{H}=ϵ\nabla H\cdot R$. Averaging this drift over an unperturbed periodic orbit is the energy balance method; a simple zero of the averaged drift selects a persistent nearby periodic orbit for sufficiently small $ϵ$. A multiple zero requires higher-order analysis.

For a nearly Hamiltonian system, the unperturbed Hamiltonian is conserved and its level curves typically contain a family of periodic orbits. A weak perturbation changes the energy slowly. The energy balance method integrates $\dot{H}$ over one unperturbed period to obtain the leading energy change $\Delta H(H)$. A zero of this change selects a possible persistent periodic orbit. A simple zero persists for sufficiently small perturbation by a Poincaré return map and the implicit function theorem. A negative slope of the averaged energy drift gives attraction; a positive slope gives repulsion. A multiple zero requires higher-order analysis, and a zero-free leading drift rules out periodic motion in the corresponding perturbative energy range.