Invariant Distributions and local theory of quasiperiodic cocycles in $\mathbb{T} ^{d} \times SU(2)$}

Abstract

We study the linear cohomological equation in the smooth category over quasi-periodic cocycles in $\mathbb{T} ^{d} \times SU(2)$. We prove that, under a full measure condition on the rotation in $\mathbb{T} ^{d}$, for a generic cocycle in an open set of cocycles, the equation admits a solution for a dense set of functions on $\mathbb{T} ^{d} \times SU(2)$ of zero average with respect to the Haar measure. This property is known as Distributional Unique Ergodicity (DUE). We then show that given such a cocycle, for a generic function no such solution exists. We thus confirm in this context a conjecture by A. Katok stating that the only dynamical systems for which the linear cohomological equation admits a smooth solution for all $0$-average functions with respect to a smooth volume are Diophantine rotations in tori. The proof is based on a careful analysis of the K.A.M. scheme of Krikorian (1999) and Karaliolios (2015), inspired by Eliasson (2002), which also gives a proof of the local density of cocycles which are reducible via finitely differentiable or measurable transfer functions.