Rigidity of the Reducibility of Gevrey Quasi-periodic Cocycles on U(n)

Abstract

We consider the reducibility problem of cocycles $(\alpha ,A)$ on $\T^d\times U(n)$ in Gevrey classes, where $\alpha$ is a Diophantine vector. We prove that, if a Gevrey cocycle is conjugated to a constant cocycle $(\alpha ,C)$ by a suitable measurable conjugacy $(0,B)$, then for almost all $C$ it can be conjugated to $(\alpha ,C)$ in the same Gevrey class, provided that $A$ is sufficiently close to a constant. If $B$ is continuous we obtain it is Gevrey smooth. We consider as well the global problem of reducibility in Gevrey classes when $d=1$.

arXiv comment47 pages, no figures

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