In earlier work in collaboration with Pavel Galashin and Thomas McConville we introduced a version of chip-firing for root systems. Our investigation of root system chip-firing led us to define certain polynomials analogous to Ehrhart polynomials of lattice polytopes, which we termed the symmetric and truncated Ehrhart-like polynomials. We conjectured that these polynomials have nonnegative integer coefficients. Here we affirm “half” of this positivity conjecture by providing a positive, combinatorial formula for the coefficients of the symmetric Ehrhart-like polynomials. This formula depends on a subtle integrality property of slices of permutohedra, and in turn a lemma concerning dilations of projections of root polytopes, which both may be of independent interest. We also discuss how our formula very naturally suggests a conjecture for the coefficients of the truncated Ehrhart-like polynomials that turns out to be false in general, but which may hold in some cases.
Revised:
Accepted:
Published online:
DOI: 10.5802/alco.79
Keywords: root system, chip-firing, Ehrhart polynomial, permutohedron, zonotope, root polytope
Hopkins, Sam 1; Postnikov, Alexander 1
@article{ALCO_2019__2_6_1159_0, author = {Hopkins, Sam and Postnikov, Alexander}, title = {A positive formula for the {Ehrhart-like} polynomials from root system chip-firing}, journal = {Algebraic Combinatorics}, pages = {1159--1196}, publisher = {MathOA foundation}, volume = {2}, number = {6}, year = {2019}, doi = {10.5802/alco.79}, zbl = {07140429}, mrnumber = {4049842}, language = {en}, url = {https://alco.centre-mersenne.org/articles/10.5802/alco.79/} }
TY - JOUR AU - Hopkins, Sam AU - Postnikov, Alexander TI - A positive formula for the Ehrhart-like polynomials from root system chip-firing JO - Algebraic Combinatorics PY - 2019 SP - 1159 EP - 1196 VL - 2 IS - 6 PB - MathOA foundation UR - https://alco.centre-mersenne.org/articles/10.5802/alco.79/ DO - 10.5802/alco.79 LA - en ID - ALCO_2019__2_6_1159_0 ER -
%0 Journal Article %A Hopkins, Sam %A Postnikov, Alexander %T A positive formula for the Ehrhart-like polynomials from root system chip-firing %J Algebraic Combinatorics %D 2019 %P 1159-1196 %V 2 %N 6 %I MathOA foundation %U https://alco.centre-mersenne.org/articles/10.5802/alco.79/ %R 10.5802/alco.79 %G en %F ALCO_2019__2_6_1159_0
Hopkins, Sam; Postnikov, Alexander. A positive formula for the Ehrhart-like polynomials from root system chip-firing. Algebraic Combinatorics, Volume 2 (2019) no. 6, pp. 1159-1196. doi : 10.5802/alco.79. https://alco.centre-mersenne.org/articles/10.5802/alco.79/
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