Beurling densities and frames of exponentials on the union of small balls
If $x_1,\dots,x_m$ are finitely many points in $\mathbb{R}^d$, let $E_\epsilon=\cup_{i=1}^m\,x_i+Q_\epsilon$, where $Q_\epsilon=\{x\in \mathbb{R}^d,\,\,|x_i|\le \epsilon/2, \, i=1,...,d\}$ and let $\hat f$ denote the Fourier transform of $f$. Given a positive Borel measure $\mu$ on $\mathbb{R}^d$, we provide a necessary and sufficient condition for the frame inequalities $$ A\,\|f\|^2_2\le \int_{\mathbb{R}^d}\,|\hat f(\xi)|^2\,d\mu(\xi)\le B\,\|f\|^2_2,\quad f\in L^2(E_\epsilon), $$ to hold for some $A,B>0$ and for some $\epsilon>0$ sufficiently small. If $m=1$, we show that the limits of the optimal lower and upper frame bounds as $\epsilon\rightarrow 0$ are equal, respectively, to the lower and upper Beurling density of $\mu$. When $m>1$, we extend this result by defining a matrix version of Beurling density. Given a (possibly dense) subgroup $G$ of $\mathbb{R}$, we then consider the problem of characterizing those measures $\mu$ for which the inequalities above hold whenever $x_1,\dots,x_m$ are finitely many points in $G$ (with $\epsilon$ depending on those points, but not $A$ or $B$). We point out an interesting connection between this problem and the notion of well-distributed sequence when $G=a\,\mathbb{Z}$ for some $a>0$. Finally, we show the existence of a discrete set $\Lambda$ such that the measure $\mu=\sum_{\lambda}\,\delta_\lambda$ satisfy the property above for the whole group $\mathbb{R}$.