Published online by Cambridge University Press: 20 November 2018
This paper characterizes when a Delone set $X$ in
${{\mathbb{R}}^{n}}$ is an ideal crystal in terms of restrictions on the number of its local patches of a given size or on the heterogeneity of their distribution. For a Delone set
$X$, let
${{N}_{X}}\left( T \right)$ count the number of translation-inequivalent patches of radius
$T$ in
$X$ and let
${{M}_{X}}\left( T \right)$ be the minimum radius such that every closed ball of radius
${{M}_{X}}\left( T \right)$ contains the center of a patch of every one of these kinds. We show that for each of these functions there is a “gap in the spectrum” of possible growth rates between being bounded and having linear growth, and that having sufficiently slow linear growth is equivalent to
$X$ being an ideal crystal.
Explicitly, for ${{N}_{X}}\left( T \right)$, if
$R$ is the covering radius of
$X$ then either
${{N}_{X}}\left( T \right)$ is bounded or
${{N}_{X}}\left( T \right)\,\ge \,T/2R$ for all
$T\,>\,0$. The constant
$1/2R$ in this bound is best possible in all dimensions.
For ${{M}_{X}}\left( T \right)$, either
${{M}_{X}}\left( T \right)$ is bounded or
${{M}_{X}}\left( T \right)\ge T/3$ for all
$T\,>\,0$. Examples show that the constant 1/3 in this bound cannot be replaced by any number exceeding 1/2. We also show that every aperiodic Delone set
$X$ has
${{M}_{X}}\left( T \right)\,\ge \,c\left( n \right)T$ for all
$T\,>\,0$, for a certain constant
$c\left( n \right)$ which depends on the dimension
$n$ of
$X$ and is
$>\,1/3$ when
$n\,>\,1$.