A Taxonomy of Crystallographic Sphere Packings

Abstract

The Apollonian circle packing, generated from three mutually-tangent circles in the plane, has inspired over the past half-century the study of other classes of space-filling packings, both in two and in higher dimensions. Recently, Kontorovich and Nakamura introduced the notion of crystallographic sphere packings, $n$-dimensional packings of spheres with symmetry groups that are isometries of $\mathbb{H}^{n+1}$. There exist at least three sources which give rise to crystallographic packings, namely polyhedra, reflective extended Bianchi groups, and various higher dimensional quadratic forms. When applied in conjunction with the Koebe-Andreev-Thurston Theorem, Kontorovich and Nakamura's Structure Theorem guarantees crystallographic packings to be generated from polyhedra in $n=2$. The Structure Theorem similarly allows us to generate packings from the reflective extended Bianchi groups in $n=2$ by applying Vinberg's algorithm to obtain the appropriate Coxeter diagrams. In $n>2$, the Structure Theorem when used with Vinberg's algorithm allows us to explore whether certain Coxeter diagrams in $\mathbb{H}^{n+1}$ for a given quadratic form admit a packing at all. Kontorovich and Nakamura's Finiteness Theorem shows that there exist only finitely many classes of superintegral such packings, all of which exist in dimensions $n\le20$. In this work, we systematically determine all known examples of crystallographic sphere packings.