Libration amplitudes of surviving orbits (in different colours) during the orbital integration of 3.4 × 10⁷ yr on the (a₀, i₀) plane. The solid line implies the libration centres for different i₀. Dozens of orbits whose libration amplitudes Δσ are larger than 300° are ignored for better visibility. Because of the scarcity of numbers, we indicate the orbits with libration amplitude Δσ above 120° with the same colour. The dashed line is the same as the solid line, but for the integration where Neptune is initially placed further away from the 2:1 MMR with Uranus (see text).