Regularity, quantitative deviation, and non-rigidity of a lacunary skew product

Abstract

Let $α$ be irrational and let $q_j$ be the denominators of its continued-fraction convergents. We study the function \[ h(x)=\sum_{j\geq1}\frac{\cos(2πq_jx)}{q_j} \] and the skew product \[f(x,y)=(x+α,y+h(x))\quad\mathrm{mod}\quad\mathbb{Z}^2.\] The function $h$ is Hölder continuous of every exponent below one. A Fourier argument shows that $h$ is not Lipschitz. The map $f$ is a toral pseudo-rotation with rotation vector $(α,0)$, but it has neither bounded mean motion nor $C^0$-rigidity. Suppose $α$ satisfies the Diophantine condition $\mathrm{DC}(τ)$. Then, $f$ has $(C,1-1/τ)$-deviation when $τ>1$; and it has $(C_δ,δ)$-deviation for every $0<δ<1$, but not for $δ=0$ when $τ=1$.