Long-term evolution over 1 Gyr of the reduced two-planet system with initial orbital parameters equal to the nominal values (hereafter IC (A0)) given in Table 1. Top: evolution of (a) the semi-axes and (b) the ecccentricities. Middle: (c) shorter interval of a₂ vs t; (d) e₂ vs t. An oscilatory motion of period ~10⁵ yr is evident. Bottom: (e) evolution of the secular angle Δϖ = ϖ₁ − ϖ₂ (blue) superposed on the evolution of the resonant angle θ₁ = 6σ₁ = 7λ₂ − λ₁ − 6ϖ₁ (green); (f) evolution of the mLCE computations (ζ) for the solved trajectories.