Dynamical maps of the 1:3 resonance on the (a, i) plane. Two indicators, the ϕ_min (left column) and the SN (right column) have been adopted. Two initial eccentricity values, e = 0.3 (top panels) and e = 0.4 (bottom panels), have been used to indicate the stability of orbits. Recognised secular mechanisms and the separatrix between the horseshoe and asymmetric resonances are plotted as lines.