Structured quantitative almost reducibility and its applications

Abstract: We establish \textit{structured quantitative almost reducibility}, tailored for analytic quasiperiodic $SL(2,\mathbb{R})$-cocycles, which effectively addresses the challenge of infinitely many \textit{normal frequency} resonances. This method paves the way for optimal arithmetic reducibility results for such cocycles, thereby resolving Jitomirskaya's conjecture. From a spectral perspective, it leads to optimal arithmetic Anderson localization for a class of quasiperiodic long-range operators on higher-dimensional lattices. In particular, using structured quantitative almost reducibility, we establish a sharp quantitative version of Aubry duality, enabling us to uncover new spectral insights for almost Mathieu operators with Diophantine frequencies. For example, we precisely determine the exponential decay rate of spectral gaps in non-critical cases, thus addressing a question raised by Goldstein. Additionally, we reveal the optimal asymptotic growth of extended eigenfunctions for subcritical almost Mathieu operators.