Stability maps in the plane (σ₀, a) for the three-body restricted problem close to the 3/1 MMR with I = 60° and Ω = 0°. For each panel, N is constant and parametrised by the initial eccentricities (from left to right): e = 0.1, e = 0.3, e = 0.5 (top), and e = 0.7, 0.9, 0.99 (bottom). The colour scale corresponds to the relative frequency diffusion index in logarithmic scale (Eq. (22)).