Ehrhart Theory of Paving and Panhandle Matroids

Ehrhart theory is a topic in geometric combinatorics which involves the enumeration of lattice points in integral dilates of polytopes.  We show that the base polytope P_M of any paving matroid M can be systematically obtained from a hypersimplex by slicing off subpolytopes. The pieces removed are base polytopes of lattice path matroids corresponding to panhandle-shaped Ferrers diagrams, whose Ehrhart polynomials we can calculate explicitly. Consequently, we can write down the Ehrhart polynomial of P_M.  Combinatorially, our construction corresponds to constructing a uniform matroid from a paving matroid by iterating the operation of stressed-hyperplane relaxation introduced by Ferroni, Nasr, and Vecchi, which generalizes the standard matroid-theoretic notion of circuit-hyperplane relaxation. We present evidence that panhandle matroids are Ehrhart positive and describe a conjectured combinatorial formula involving chain gangs and Eulerian numbers from which Ehrhart positivity of panhandle matroids will follow. As an application of the main result, we calculate the Ehrhart polynomials of matroids associated with Steiner systems and finite projective planes, and show that they depend only on their design-theoretic parameters. (This is joint work with D. Hanely, J. Martin, D. McGinnis, D. Miyata, G. Nasr, and, M. Yin).