Low-energy dynamics in generic potential fields: Hyperbolic periodic orbits and non-ergodicity

We prove that, on each low energy level, the natural Hamiltonian system defined by a generic smooth potential on ${\mathbf{T}}^2$ exhibits an arbitrarily high number of hyperbolic periodic orbits and a positive-measure set of invariant tori. Hence, quasi-periodic motion and hyperbolic behavior typically coexist in the low-energy dynamics of natural Hamiltonian systems with two degrees of freedom.