Half-open integer parallelepipeds and polytope Dedekind sums

Abstract

We study the Ehrhart theory of half-open $d$-dimensional integer parallelepipeds $\Pi$. Although the lattice-point count $t\Pi \cap {\mathbb{Z}}^d$ is known to be simply $\vol Πt^d$ for positive integer $t$, the corresponding counting function for arbitrary real dilations $t$ has subtle, nontrivial periodic structure. We give explicit formulas for this real Ehrhart quasi-polynomial, and more generally for all the discrete moments of the real dilates of $\Pi$: ${\sum }_{p\in t\Pi \cap {\mathbb{Z}}^d}⟨p,z{⟩}^m$. The formulas are expressed in terms of Barnes polynomials and polytope Dedekind sums, which encode the periodic lattice flow of translated integer lattices on the flat torus determined by $\Pi$. Our approach develops further the study of polytope Dedekind sums, introduced recently in \cite{Robins2026}. In particular, we obtain novel identities for polytope Dedekind sums by using iterated discrete derivatives. Moreover, we show that the Ehrhart quasi-coefficients of $L_{\Pi }(t)$ are precisely alternating sums of polytope Dedekind sums. Finally, we give an Ehrhart-type reciprocity law relating $L_{\Pi }(t)$ at negative arguments to the lattice-point count of the `opposite' half-open parallelepiped.

25 pages, 1 figure