Complex sentence: The KAM theorem is usually stated in terms of trajectories in phase space of an integrable Hamiltonian system that are confined to a doughnut-shaped surface, an invariant torus.
Split into simpler sentences: The KAM theorem is usually stated in terms of trajectories in phase space of an integrable Hamiltonian system. The motion of an integrable system is confined to a doughnut-shaped surface, an invariant torus.