Phase-space structure for two- and three-sine-wave initial conditions at the collapse time: Q1D-2SIN (top left), ANI-2SIN (centre left), SYM-2SIN (bottom left), Q1D-3SIN (top right), ANI-3SIN (centre right), and SYM-3SIN (bottom right). The intersection of the phase-space sheet with the y = 0 plane for two sine waves and y = z = 0 hyper-plane for three sine waves is displayed in (x, υ_x) subspace. Simulation results are compared with standard LPT predictions, which are supplemented with the blue line, denoted by’ΈΧΤ’, which corresponds to the formal extrapolation to the infinite order proposed by STC18 and sketched in Sect. 4.1 for the collapse time. For completeness, the quasi-1D approach (Rampf & Frisch 2017), denoted by Q1D, is also presented (see Appendix C for details).