A Discrete Quadratic Carleson Theorem on $ \ell ^2 $ with a Restricted Supremum

Abstract

Consider the discrete maximal function acting on $\ell^2(\mathbb Z)$ functions \[ \mathcal{C}_Λ f( n ) := \sup_{ λ\in Λ} \left| \sum_{m \neq 0} f(n-m) \frac{e^{2 πiλm^2}} {m} \right| \] where $Λ\subset [0,1]$. We give sufficient conditions on $Λ$, met by certain kinds of Cantor sets, for this to be a bounded sublinear operator. This result is a discrete analogue of E. M. Stein's integral result, that the maximal operator below is bounded on $L^2(\mathbb R)$. \[ \mathcal{C}_2 f(x):= \sup_{λ\in \mathbb R} \left| \int f(x-y) \frac{e^{2πi λy^2}}{y} \ dy \right|.\] The proof of our result relies heavily on Bourgain's work on arithmetic ergodic theorems, with novel complexity arising from the oscillatory nature of the question at hand, and difficulties arising from the the parameter $λ$ above.