x component of force as a function of x inside and in the vicinity of a pancake seeded by three sine waves in the context of LPT dynamics, with −1.8 ≤ x/x_max ≤ 1.8, where x_max is the maximum extension of the caustic region along the x-axis, as given by Eq. (44) in our approximate formalism. The output time is set to Δ = 0.001 with synchronisation (see Eq. (53)). In each panel, the curves of various colours correspond to different values of y (and z = 0 in the 3D case), namely y/y_max = 0 (orange), 0.75 (green), and 1.5 (red, outside the multi-stream region), where y_max is the maximum extension of the caustic region along the y-axis (Eq. (45)). From top to bottom, we consider Q1D-2SIN, ANI-2SIN, Q1D-3SIN, and ANI-3SIN. From left to right, the predictions are made with 5, 20 (10), and 40LPT (15LPT) for two-sine-wave (three-sine-wave) initial conditions. The dashed and solid lines respectively represent the ‘exact’ force given by Eq. (14) and the analytic prediction (51).