4D PSS ($y,\dot{y},z,\dot{z}$) for x = 0 and $\dot{x}<0$, for energy level E_j = −4.5. (a) For smaller radii. (b) For larger radii. The colour gradient allows the visualization of the fourth dimension, ż, in rotational tori of regular orbits according to the colour bar on the left (minimum ż corresponds to 0 and maximum ż to 1). The rotational torus of the regular ring-like orbit is also shown. The 2D PSS of the planar orbits is plotted in black and the unstable manifolds of the planar unstable periodic orbits in the chaotic region are in red. 3D sticky chaotic orbits (green) escape from the system after very long times. (a′) and (b′) are the 2D projections (y,ẏ) of the 4D PSS of the same orbits as in (a) and (b).