Homology and carrier faces of simplicial distributions

Abstract

We study the facial structure of simplicial distribution polytopes using topological and algebraic methods. In the theory of simplicial distributions, measurement and event spaces are represented by simplicial sets, and compatible probability assignments form a convex polytope. Given a simplicial distribution, it is known that its supports determine its carrier face. Moreover, the supports form a simplicial subset of the event space. This naturally raises the question of how the topology of this simplicial subset is related to the corresponding carrier face. Our main objective is to relate the dimension of the carrier face to the topology and combinatorial structure of the space of supports. For $n$-facet-paired measurement spaces, we first express the carrier dimension in terms of the nullity of an associated facet matrix. When the measurement space is additionally orientable, we show that the carrier dimension is determined by the top-dimensional homology of the space of supports, yielding, in particular, a homological criterion for detecting vertices. We then develop an algebraic reduction based on a strong connectivity relation on the generating simplices of the space of supports. This construction yields a reduced facet matrix, indexed by the strongly connected components, whose null space is isomorphic to that of the original facet matrix. As a consequence, both the carrier dimension and extremality can be determined from the topology of the support and the way its strongly connected components are glued together.