Convergence behaviour of the shell-crossing time. Top: shell-crossing time calculated at nth-order LPT (dots), together with the fitting function given by Eq. (34) (solid lines), as a function of the perturbation order for various initial conditions, as indicated in the panel. The dashed horizontal lines present the values estimated from the simulations (see Appendix B.1), corresponding to the sixth column of Table 1. Bottom: relative error between the shell-crossing time calculated with nth-order LPT and the one obtained with the fitting formula.