Georgia Institute of Technology College of Sciences

For $c\in(1,2)$ we consider the following operators
$$\mathcal{C}_{c}f(x) \colon = \sup_{\lambda \in [-1/2,1/2)} \bigg| \sum_{n \neq 0} f(x-n) \frac{e^{2\pi i\lambda \lfloor |n|^{c} \rfloor}}{n} \bigg|\text{,}$$
$$\mathcal{C}^{\mathsf{sgn}}_{c}f(x) \colon = \sup_{\lambda \in [-1/2,1/2)} \bigg| \sum_{n \neq 0} f(x-n) \frac{e^{2\pi i\lambda \mathsf{sign}(n) \lfloor |n|^{c} \rfloor}}{n} \bigg| \text{,}$$
and prove that both extend boundedly on $\ell^p(\mathbb{Z})$, $p\in(1,\infty)$. 

The second main result is establishing almost everywhere pointwise convergence for the following ergodic averages
$$A_Nf(x)\colon =\frac{1}{N}\sum_{n=1}^N f(T^n S^{\lfloor n^c\rfloor} x) \text{,}$$
where $T,S\colon X\to X$ are commuting measure-preserving transformations on a  $\sigma$-finite measure space $(X,\mu)$, and $f\in L_{\mu}^p(X), p\in(1,\infty)$. 

The point of departure for both proofs is the study of exponential sums with phases  $\xi_2 \lfloor |n^c|\rfloor+ \xi_1n$ through the use of a simple variant of the circle method.

This talk is based on joint work with Leonidas Daskalakis.