Tests of the ballistic approximation in Vlasov simulations: measured phase-space structure for two-and three-sine-wave initial conditions. This figure is analogous to Fig. 2, except that it shows a zoomed-in view of the central part of the system shortly after collapse. We test the validity of the ballistic approximation by using two simulation snapshots slightly before (red curves, with as shown in Table 1 except for ϵ_3D = (1,1), for which a = 0.03103) and slightly after the collapse time (black, with a = a_sc + Δa). The ballistic approximation, as described in the main text, is applied to the red curves to obtain the magenta ones, to be compared directly with the exact solution, in black, given by the simulation.