KAM theory for almost-periodic equilibria in one-dimensional almost-periodic media

We consider one-dimensional chains of interacting particles subjected to one-dimensional almost-periodic media, which is a rather general model. We formulate and prove KAM type theorems corresponding to both short-range and long-range interactions respectively. All theorems presented have an a posteriori format and establish the existence of almost-periodic equilibria. The new part here is that the interaction is long-range and the potential function is given by some almost-periodic function with infinitely many incommensurate frequencies. In both cases, we do not need to assume that the system is close to integrable. We will show that if there exists an approximate solution for the functional equations, which satisfies some appropriate non-degeneracy conditions, then a true solution nearby is obtained. This procedure may be used to validate efficient numerical computations. Furthermore, to well understand the role of almost-periodic media which can be approximated by quasi-periodic ones, we present different approaches—direct infinite-dimensional scheme and step by step increase of complexity method—to the study of the above results of the almost-periodic models.