An Extremal Property of the Hexagonal Lattice

Abstract

We describe an extremal property of the hexagonal lattice $\Lambda \subset \mathbb{R}^2$. Let $p$ denote the circumcenter of its fundamental triangle (a so-called deep hole) and let $A_r$ denote the set of lattice points that are at distance $r$ from $p$ \begin{equation} A_r = \left\{ \lambda \in \Lambda: \| \lambda - p \| = r\right\}. \end{equation} If $\Gamma$ is a small perturbation of $\Lambda$ in the space of lattices with fixed density and $C_r$ denotes the set of points in $A_r$ shifted to the new lattice, then \begin{equation} \sum_{\mu \in C_r}{ \| p - \mu\|} - \sum_{\lambda \in A_r}{ \| p - \lambda\|} \gtrsim r \, |A_r| \, d(\Lambda, \Gamma)^2, \end{equation} where $d(\Lambda, \Gamma)$ denotes the distance between the lattices: the hexagonal lattice has the property that `far away points are closer than they are for nearby lattices'. This has implications in the calculus of variations: assume \begin{equation} g_{\Gamma}(z) = \sum_{\gamma \in \Gamma} f( \|z - \gamma \|) \quad \mbox{ satisfies } \quad \min_{z \in \mathbb{R}^2} g_{\Lambda}(z) = g_{\Lambda}(p). \end{equation} For a certain class of compactly supported functions $f$, the hexagonal lattice $\Lambda$ is then a strict local maximizer of \begin{equation} \max_{\Gamma} \min_{z \in \mathbb{R}^2} \sum_{\gamma \in \Gamma}{f( \|z - \gamma\| )}, \end{equation} where the maximum runs over all lattices of fixed density.