Inscribable fans I: Inscribed cones and virtual polytopes

TL;DR

This paper studies polytopes inscribed in spheres, their associated cones, and virtual polytopes, revealing their structure, classification, and connections to hyperbolic geometry, Delaunay subdivisions, and reflection groupoids.

Contribution

It introduces inscribed cones and virtual polytopes, analyzes their structure, classification, and polynomial-time decidability of inscribability, and links them to hyperbolic and geometric group theory.

Findings

Inscribed cones are closed under Minkowski addition.

Polynomial-time algorithm for testing inscribability.

Classification of inscribable fans and polytopes in low dimensions.

Abstract

We investigate polytopes inscribed into a sphere that are normally equivalent (or strongly isomorphic) to a given polytope $P$. We show that the associated space of polytopes, called the inscribed cone of $P$, is closed under Minkowski addition. Inscribed cones are interpreted as type cones of ideal hyperbolic polytopes and as deformation spaces of Delaunay subdivisions. In particular, testing if there is an inscribed polytope normally equivalent to $P$ is polynomial time solvable. Normal equivalence is decided on the level of normal fans and we study the structure of inscribed cones for various classes of polytopes and fans, including simple, simplicial, and even. We classify (virtually) inscribable fans in dimension $2$ as well as inscribable permutahedra and nestohedra. A second goal of the paper is to introduce inscribed virtual polytopes. Polytopes with a fixed normal fan $\mathcal{N}$ form a monoid with respect to Minkowski addition and the associated Grothendieck group is called the type space of $\mathcal{N}$. Elements of the type space correspond to formal Minkowski differences and are naturally equipped with vertices and hence with a notion of inscribability. We show that inscribed virtual polytopes form a subgroup, which can be non-trivial even if $\mathcal{N}$ does not have actual inscribed polytopes. We relate inscribed virtual polytopes to routed particle trajectories, that is, piecewise-linear trajectories of particles in a ball with restricted directions. The state spaces gives rise to connected groupoids generated by reflections, called reflection groupoids. The endomorphism groups of reflection groupoids can be thought of as discrete holonomy groups of the trajectories and we determine when they are reflection groups.