The existence of an invariant surface in high-dimensional systems greatly
influences the behavior in a neighborhood of the invariant surface. We
prove theorems that predict the behavior of periodic orbits in the
vicinity of an invariant surface on which the motion is conjugate to a
Diophantine rotation for symplectic maps and quasiperiodic
perturbations of symplectic maps.
Our results allow for efficient numerical algorithms that can
serve as an indication for the breakdown of invariant surfaces.