Stathis Tompaidis Approximation of invariant surfaces by periodic orbits in high-dimensional maps. Some rigorous results. (64K, TeX) ABSTRACT. 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 explain the behavior of periodic orbits in the vicinity of an invariant surface for symplectic maps and quasi-periodic perturbations of symplectic maps. Our results allow for efficient numerical algorithms that can serve as an indication for the breakdown of invariant surfaces.