Trajectories of in the complete Hamiltonian (3) for the six iterations needed for the algorithm to converge to the libration centre. Iteration 0 is the equilibrium (main branch) of the simplified Hamiltonian (25) at δ = 5. It is rather far from the libration centre of the complete Hamiltonian (see also Fig. 4). After six iterations, the algorithm converges to the libration centre and the motion is periodic (hence the closed curves) with frequency ν₂ (see Sect. 2.2.1). The planetary masses are the same as in Table 1 and the resonance chain is 1:1:2.