Delone sets generated by square roots

Abstract

Delone sets are locally finite point sets, such that (a) any two points are separated by a given minimum distance, and (b) there is a given radius so that every ball of that radius contains at least one point. Important examples include the vertex set of Penrose tilings and other regular model sets, which serve as a mathematical model for quasicrystals. In this note we show that the point set given by the values $\sqrt{n} e^{2πi α\sqrt{n}}$ with $n=1,2,3,\ldots$ is a Delone set in the complex plane, for any $α>0$. This complements Akiyama's recent observation (see arXiv:1904.10815) that $\sqrt{n} e^{2πi α{n}}$ with $n=1,2,3,\ldots$ forms a Delone set, if and only if $α$ is badly approximated by rationals. A key difference is that our setting does not require Diophantine conditions on $α$.