Education
In addition to POHEA, ...
The subsections here, accessible from the side-bar, will guide you toward other educational materials that we have developed and used over the years...
Example (QT movie): Mathcad was used to make this animation of a distribution of particles in phase space under the influence of a thin sextupole magnet in an otherwise linear system. As the tune approaches the 1/3 integer, the stable phase space region shrinks and particles are "extracted."