The Simplest Flexible Cross-Polytopes
According to R. Bricard there exist three types of flexible octahedra. The octahedra of Type 3 are unsymmetric and admit two flat poses. With regard to higher-dimensional analogues of octahedra called cross-polytopes, A.A. Gaifullin presented in 2014 a complete classification of flexible types in n-dimensional Euclidean, hyperbolic and spherical spaces for n>3. The goal of this presentation is a synthetic approach to a particular family in the Euclidean n-space, the flexible cross-polytopes that admit two poses within hyperplanes. We provide a construction of their flat poses and prove several properties of these higher-dimensional analogues to Bricard's type-3 octahedra. According to Gaifullin, they are the simplest from the algebraic point of view.