“…Then system (9) has the periodic solution of period T ε (u(t, ε), v(t, ε), w(t, ε)) = (r(t, ε) cos θ(t, ε), r(t, ε) sin θ(t, ε), w(t, ε)) , for ε sufficiently small, with (u(0, ε), v(0, ε), w(0, ε)) = (r i , 0, w i ). Now we apply the change of variables (8) to this one and obtain for small ε the periodic solution (X(t, ε), Y (t, ε), Z(t, ε)) of system (7) with the same period, such that (X(0, ε, Y (0, ε), Z(0, ε)) = (w i + r i /δ, r i , 0). Finally, for ε = 0 sufficiently small, system (6) has the periodic solution (x(t, ε), y(t, ε), z(t, ε)) = (εX(θ), εY (θ), εZ(θ)), with (x(0, ε), y(0, ε), z(0, ε)) = ε(w i +r i /δ, r i , 0), and that clearly tends to the origin of coordinates when ε → 0.…”