Locations of the fourth-degree secular resonances for Trojans around the L₄ point on the (a₀, i₀) plane. The resonances are labelled (see text for the meaning) along the curves. In consideration of the frequency drift of the inner planets, the dashed lines in the left panel delimit the possible locations of S4_A and G4_F (see Sect. 4.5). The black and red dashed lines represent the results derived from the data taken from Laskar (1990) and our integration, respectively.