Embedded flexible spherical cross-polytopes with non-constant volumes

TL;DR

This paper constructs embedded flexible cross-polytopes in spheres of all dimensions, demonstrating non-constant volume during flexion and providing counterexamples to the Bellows Conjecture in spherical geometry.

Contribution

It presents the first known embedded flexible polyhedra in dimensions 4 and higher and introduces a modified Bellows Conjecture for spherical polyhedra.

Findings

Volumes of the constructed cross-polytopes are non-constant during flexion.

Counterexamples to the Bellows Conjecture are provided in spherical geometry.

Relations on face volumes of flexible cross-polytopes are established.

Abstract

We construct examples of embedded flexible cross-polytopes in the spheres of all dimensions. These examples are interesting from two points of view. First, in dimensions 4 and higher, they are the first examples of embedded flexible polyhedra. Notice that, unlike in the spheres, in the Euclidean spaces and the Lobachevsky spaces of dimensions 4 and higher, still no example of an embedded flexible polyhedron is known. Second, we show that the volumes of the constructed flexible cross-polytopes are non-constant during the flexion. Hence these cross-polytopes give counterexamples to the Bellows Conjecture for spherical polyhedra. Earlier a counterexample to this conjecture was built only in dimension 3 (Alexandrov, 1997), and was not embedded. For flexible polyhedra in spheres we suggest a weakening of the Bellows Conjecture, which we call the Modified Bellows Conjecture. We show that this conjecture holds for all flexible cross-polytopes of the simplest type among which there are our counterexamples to the usual Bellows Conjecture. By the way, we obtain several geometric results on flexible cross-polytopes of the simplest type. In particular, we write relations on the volumes of their faces of codimensions 1 and 2.