Quantitative estimates in Beurling--Helson type theorems

Abstract

We consider the spaces $A_p(\mathbb T)$ of functions $f$ on the circle $\mathbb T$ such that the sequence of Fourier coefficients $\fu{\f}=\{\fu{\f}(k), ~k \in \mathbb Z\}$ belongs to $l^p, ~1\leq p<2$. The norm on $A_p(\mathbb T)$ is defined by $\|f\|_{A_p}=\|\fu{\f}\nolinebreak\|_{l^p}$. We study the rate of growth of the norms $\|e^{i\lambda\varphi}\|_{A_p}$ as $|\lambda|\rightarrow \infty, ~\lambda\in\mathbb R,$ for $C^1$ -smooth real functions $\varphi$ on $\mathbb T$. The results have natural applications to the problem on changes of variable in the spaces $A_p(\mathbb T)$.