Lagrangian combinatorics of matroids

The Lagrangian geometry of matroids was introduced in [2] through the construction of the conormal fan of a matroid M. We used the conormal fan to give a Lagrangian-geometric interpretation of the h-vector of the broken circuit complex of M: its entries are the degrees of the mixed intersections of certain convex piecewise linear functions γ and δ on the conormal fan of M. By showing that the conormal fan satisfies the Hodge-Riemann relations, we proved Brylawski’s conjecture that this h-vector is a log-concave sequence. This sequel explores the Lagrangian combinatorics of matroids, further developing the combinatorics of biflats and biflags of a matroid, and relating them to the theory of basis activities developed by Tutte, Crapo, and Las Vergnas. Our main result is a combinatorial realization of the intersection-theoretic computation above: we write the k-th mixed intersection of γ and δ explicitly as a sum of biflags corresponding to the nbc bases of internal activity k + 1.