Geometric bijections for regular matroids, zonotopes, and Ehrhart theory

Let M be a regular matroid. The Jacobian group Jac(M) of M is a finite abelian group whose cardinality is equal to the number of bases of M. This group generalizes the definition of the Jacobian group (also known as the critical group or sandpile group) Jac(G) of a graph G (in which case bases of the corresponding regular matroid are spanning trees of G). There are many explicit combinatorial bijections in the literature between the Jacobian group of a graph Jac(G) and spanning trees. However, most of the known bijections use vertices of G in some essential way and are inherently “non-matroidal”. In this work, we construct a family of explicit and easy-to-describe bijections between the Jacobian group of a regular matroid M and bases of M, many instances of which are new even in the case of graphs. We first describe our family of bijections in a purely combinatorial way in terms of orientations; more specifically, we prove that the Jacobian group of M admits a canonical simply transitive action on the set G(M) of circuit-cocircuit reversal classes of M, and then define a family of combinatorial bijections ß_s,s* between G(M) and bases of M. (Here s (resp s^*) is an acyclic signature of the set of circuits (resp. cocircuits) of M.) We then give a geometric interpretation of each such map ß = ß_s,s* in terms of zonotopal subdivisions which is used to verify that ß is indeed a bijection. Finally, we give a combinatorial interpretation of lattice points in the zonotope Z; by passing to dilations we obtain a new derivation of Stanley's formula linking the Ehrhart polynomial of Z to the Tutte polynomial of M.