A note on a sparse sampling conjecture
A conjecture made by the second author was that two convex, centrally symmetric bodies of positive measure which are not multi-tilers must agree up to a rigid motion whenever the Fourier transforms of their indicator functions agree on \(\Z^d\). Counterexamples were constructed by the first author in dimensions \(d\geq4\), showing that the lattice \(\Z^d\) can be too sparse. Here we show that the initial conjecture is also false in the remaining dimensions \(2\) and \(3\). However, we prove a related positive result, with a stronger conclusion and without either central symmetry or the non-multitiling assumption. Let \(\mathcal L\subset\R^d\) be a full-rank lattice, and let \(\mathcal P,Q\subset\R^d\) be connected finite unions of convex bodies such that no two distinct points of either set are congruent modulo \(\mathcal L\). If the Fourier transforms of their indicator functions agree on the dual lattice \(\mathcal L^*\), then \(Q=\mathcal P+\ell\) for some \(\ell\in \mathcal L\). In particular, if both sets are symmetric about the origin, then \(\mathcal P=Q\).