Euler Constraints and the Cubic Criticality of Complete Bipartite Preference Structures

Abstract

We introduce a polyhedral realizability problem for complete bipartite preference structures, the two-sided strict preference profiles that underlie the stable marriage problem. Given two parts M and W of size k, a strong polyhedral realization asks for a bipartite polyhedral graph with bipartition classes of size k in which every vertex is adjacent to all but one vertex on the opposite side. This condition attempts to represent complete opposite-side preference capacity by direct polyhedral adjacency, with one geometrically exceptional opposite-side vertex for each agent. We prove an Euler-type criticality theorem: such a strong polyhedral realization exists if and only if k=4, and in that case the underlying graph is the cube graph Q_3, equivalently K_{4,4} minus a perfect matching. The proof is the collision of the strong edge requirement E=k(k-1) with the bipartite planar bound E<=4k-4 and the minimum-degree constraint for polyhedral graphs. We then distinguish weak realizations. A family of (k-1)-gonal trapezohedra gives balanced bipartite polyhedral graphs with 2k vertices and 4k-4 edges for all k>=4, showing that weak maximal realizability persists beyond the cubic critical case. However, the Euler interval 3k<=E<=4k-4 is not fully realizable: we give a direct proof that no bipartite polyhedral graph exists with V=10 and E=15, equivalently that there is no even triangulation of the sphere on seven vertices. Finally, in the cube case, we show that cube-distance compatibility of a size-four preference profile is equivalent to the existence of a perfect matching of mutually last-ranked pairs. This gives a first preference-theoretic manifestation of the cubic criticality.