Libration frequency f of the 1:1 resonant angle for VTs (cos σ) vs initial semi-major axis (bottom abscissa) and initial inclination (top abscissa). Solid squares in orange indicate the orbits with varying semi-major axis but fixed initial inclination i₀ = 17°; while open circles in blue and diamonds in green indicate the orbits with varying initial inclination but fixed semi-major axes a₀ = 0.72053 and 0.72143 AU, respectively. The frequency of the 13:8 NMMR between the Earth and Venus f_13:8 and its double value 2f_13:8 are depicted by the dashed and solid lines, respectively. The arrows point to the intersections of f with these two lines.