Ehrhart theory of paving and panhandle matroids. (English) Zbl 1526.05028

Summary: We show that the base polytope \(P_M\) of any paving matroid \(M\) can be systematically obtained from a hypersimplex by slicing off certain subpolytopes, namely base polytopes of lattice path matroids corresponding to panhandle-shaped Ferrers diagrams. We calculate the Ehrhart polynomials of these matroids and consequently write down the Ehrhart polynomial of \(P_M\), starting with Katzman’s formula for the Ehrhart polynomial of a hypersimplex. The method builds on and generalizes L. Ferroni’s work on sparse paving matroids [Discrete Comput. Geom. 68, No. 1, 255–273 (2022; Zbl 1490.05026)]. Combinatorially, our construction corresponds to constructing a uniform matroid from a paving matroid by iterating the operation of stressed-hyperplane relaxation introduced by L. Ferroni [“Stressed hyperplanes and Kazhdan-Lusztig gamma-positivity for matroids”, Preprint, arXiv:2110.08869], 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 forests 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: for example, while projective planes of the same order need not have isomorphic matroids, their base polytopes must be Ehrhart equivalent.


05B35 Combinatorial aspects of matroids and geometric lattices
05C31 Graph polynomials
52B40 Matroids in convex geometry (realizations in the context of convex polytopes, convexity in combinatorial structures, etc.)
05B25 Combinatorial aspects of finite geometries


Zbl 1490.05026


SageMath; OEIS
Full Text: DOI arXiv


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