# ALGEBRAIC COMBINATORICS

Three Ehrhart quasi-polynomials
Algebraic Combinatorics, Volume 2 (2019) no. 3, p. 379-416

Let $𝔭\left(b\right)\subset {ℝ}^{d}$ be a semi-rational parametric polytope, where $b=\left({b}_{j}\right)\in {ℝ}^{N}$ is a real multi-parameter. We study intermediate sums of polynomial functions $h\left(x\right)$ on $𝔭\left(b\right)$,

${S}^{L}\left(𝔭\left(b\right),h\right)=\sum _{y}{\int }_{𝔭\left(b\right)\cap \left(y+L\right)}h\left(x\right)\phantom{\rule{0.166667em}{0ex}}\mathrm{d}x,$

where we integrate over the intersections of $𝔭\left(b\right)$ with the subspaces parallel to a fixed rational subspace $L$ through all lattice points, and sum the integrals. The purely discrete sum is of course a particular case ($L=0$), so ${S}^{0}\left(𝔭\left(b\right),1\right)$ counts the integer points in the parametric polytopes.

The chambers are the open conical subsets of ${ℝ}^{N}$ such that the shape of $𝔭\left(b\right)$ does not change when $b$ runs over a chamber. We first prove that on every chamber of ${ℝ}^{N}$, ${S}^{L}\left(𝔭\left(b\right),h\right)$ is given by a quasi-polynomial function of $b\in {ℝ}^{N}$. A key point of our paper is an analysis of the interplay between two notions of degree on quasi-polynomials: the usual polynomial degree and a filtration, called the local degree.

Then, for a fixed $k\le d$, we consider a particular linear combination of such intermediate weighted sums, which was introduced by Barvinok in order to compute efficiently the $k+1$ highest coefficients of the Ehrhart quasi-polynomial which gives the number of points of a dilated rational polytope. Thus, for each chamber, we obtain a quasi-polynomial function of $b$, which we call Barvinok’s patched quasi-polynomial (at codimension level $k$).

Finally, for each chamber, we introduce a new quasi-polynomial function of $b$, the cone-by-cone patched quasi-polynomial (at codimension level $k$), defined in a refined way by linear combinations of intermediate generating functions for the cones at vertices of $𝔭\left(b\right)$.

We prove that both patched quasi-polynomials agree with the discrete weighted sum $b↦{S}^{\left\{0\right\}}\left(𝔭\left(b\right),h\right)$ in the terms corresponding to the $k+1$ highest polynomial degrees.

Revised : 2018-10-25
Accepted : 2018-11-01
Published online : 2019-06-06
DOI : https://doi.org/10.5802/alco.46
Classification:  05A15,  52C07,  68R05,  68U05,  52B20
Keywords: Ehrhart polynomials, generating functions, Barvinok’s algorithm, parametric polytopes
@article{ALCO_2019__2_3_379_0,
author = {Baldoni, Velleda and Berline, Nicole and De Loera, Jes\'us A. and K\"oppe, Matthias and Vergne, Mich\ele},
title = {Three Ehrhart quasi-polynomials},
journal = {Algebraic Combinatorics},
publisher = {MathOA foundation},
volume = {2},
number = {3},
year = {2019},
pages = {379-416},
doi = {10.5802/alco.46},
zbl = {07066881},
language = {en},
url = {https://alco.centre-mersenne.org/item/ALCO_2019__2_3_379_0}
}

Three Ehrhart quasi-polynomials. Algebraic Combinatorics, Volume 2 (2019) no. 3, pp. 379-416. doi : 10.5802/alco.46. https://alco.centre-mersenne.org/item/ALCO_2019__2_3_379_0/`

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