Beurling density theorems for sampling and interpolation on the flat cylinder

Abstract

We consider the Fock space weighted by $e^{-α|z|^{2}}$, of entire and quasi-periodic (modulo a weight dependent on $ν$) functions on ${C}$. The quotient space $\mathbb{C}/\mathbb{Z}$, called `The flat cylinder', is represented by the vertical strip $[0,1)\times \mathbb{R}$, which tiles ${C}$ by ${Z}$-translations and is therefore a fundamental domain for $\mathbb{C}/\mathbb{Z}$. Our main result gives a complete characterization of the sets $Z\subset Λ\left( \mathbb{Z}\right) $ that are sets of sampling or interpolation, in terms of concepts of upper and lower Beurling densities, $ D^{+}(Z)$ and $D^{-}(Z)$, adapted to the geometry of $\mathbb{C}/\mathbb{Z}$. The critical `Nyquist density' is the real number $\frac{α}{π}$, meaning that the condition $D^{-}(Z)>\frac{α}{π}$ characterizes sets of sampling, while the condition $D^{+}(Z)<\frac{α}{π}$ characterizes sets of interpolation. The results can be reframed as a complete characterization of Gabor frames and Riesz basic sequences (given by arbitrary discrete sets in $Z\subset Λ\left( \mathbb{Z}\right) $), with time-periodized Gaussian windows (theta-Gaussian), for spaces of functions $f$, measurable in $\mathbb{R}$, square-integrable in $(0,1)$, and quasi-periodic with respect to integer translations.