Van der Corput inequality for real line and Wiener-Wintner theorem for amenable groups

Abstract

We extend the classical van der Corput inequality to the real line. As a consequence, we obtain a simple proof of the Wiener-Wintner theorem for the $\mathbb{R}$-action which assert that for any family of maps $(T_t)_{t \in \mathbb{R}}$ acting on the Lebesgue measure space $(\Omega,{\cal {A}},\mu)$ where $\mu$ is a probability measure and for any $t\in \mathbb{R}$, $T_t$ is measure-preserving transformation on measure space $(\Omega,{\cal {A}},\mu)$ with $T_t \circ T_s =T_{t+s}$, for any $t,s\in \mathbb{R}$. Then, for any $f \in L^1(\mu)$, there is a a single null set off which $\displaystyle \lim_{T \rightarrow +\infty} \frac1{T}\int_{0}^{T} f(T_t\omega) e^{2 i \pi \theta t} dt$ exists for all $\theta \in \mathbb{R}$. We further present the joining proof of the amenable group version of Wiener-Wintner theorem due to Weiss and Ornstein.