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 $P\neq \lambda\Delta_{2k,k}\neq Q$. Since $\Delta_{2k,k}$ is a deformed permutahedron, 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.