“…Averaging method [32,35] is suitable for this purpose. Indeed, it has been extensively used to determine periodic orbits in both non rotating [1,5,6,[18][19][20] and rotating potentials [8,9] and also to establish periodic orbits for system (1), as it is done in [17]. The method is simple to apply, although some times tricky, but, if the system is Hamiltonian, alternative techniques can be used.…”