Abstract. We prove the existence and local uniqueness of invariant tori on the verge of breakdown for two systems: the quasi-periodically driven logistic map and the quasi-periodically forced standard map. These systems exemplify two scenarios: the Heagy-Hammel route for the creation of strange nonchaotic attractors and the nonsmooth bifurcation of saddle invariant tori. Our proofs are computerassisted and are based on a tailored version of the Newton-Kantorovich theorem. The proofs cannot be performed using classical perturbation theory because the two scenarios are very far from the perturbative regime, and fundamental hypotheses such as reducibility or hyperbolicity either do not hold or are very close to failing. Our proofs are based on a reliable computation of the invariant tori and a careful study of their dynamical properties, leading to the rigorous validation of the numerical results with our novel computational techniques.
Key words. normally hyperbolic invariant manifolds, computer validations, invariant tori, strange nonchaotic attractors
AMS subject classifications. 37C55, 37D10, 65P99DOI. 10.1137/1008092221. Introduction. The goal of this paper is to present a new methodology to provide rigorous proofs of the existence and local uniqueness of (fiberwise hyperbolic) invariant tori in quasi-periodic systems, even in cases in which the available perturbative theory does not apply. The methodology is suitable for computer-assisted proofs and consists in checking the hypotheses of a validation result based on the Newton-Kantorovich theorem [27]. As an application of the methodology, we prove the existence and local uniqueness of invariant tori on the verge of breakdown in two scenarios: the Heagy-Hammel route to strange nonchaotic attractors (SNA) [32] in a quasi-periodically driven logistic map and the breakdown of saddle tori [26] in a quasi-periodically forced standard map.