“…If ε = 0, the linearized system at the equilibrium point E 1 is always non resonant, i.e., the eigenvalues of A 1 are never of the type ik, with k ∈ Z. Therefore, since, for j = 1, 2, the functions Γ j (s, ρ, η, ϕ, v; ε) are locally Lipschitz continuous in (ρ, η, ϕ, v), a classical perturbation theorem applies (see, e.g., [10, Chapter 14, Theorem 1.1]), providing the existence of a 2π-periodic solution of (8), for ε small enough. Correspondingly, by the changes of variables made above, we have a T -periodic solution of (1).…”