Indecomposability of the median hypersimplex and polytopality of the hemi-icosahedral Bier sphere

Abstract

We prove that the median hypersimplex ${\Delta }_{2k,k}$ is Minkowski indecomposable, i.e. it cannot be expressed as a non-trivial Minkowski sum ${\Delta }_{2k,k}=P+Q$, where $P\ne \lambda {\Delta }_{2k,k}\ne Q$. We obtain as a corollary that ${\Delta }_{2k,k}$ represents a ray in the submodular cone (the deformation cone of the permutahedron). Building on the previously developed geometric methods and extensive computer search, we exhibit a twelve vertex, $4$-dimensional polytopal realization of the Bier sphere of the hemi-icosahedron, the vertex minimal triangulation of the real projective plane.