On a class of power ideals

Abstract

In this paper we study the class of power ideals generated by the $k^n$ forms $(x_0+\xi^{g_1}x_1+\ldots+\xi^{g_n}x_n)^{(k-1)d}$ where $\xi$ is a fixed primitive $k^{th}$-root of unity and $0\leq g_j\leq k-1$ for all $j$. For $k=2$, by using a $\mathbb{Z}_k^{n+1}$-grading on $\mathbb{C}[x_0,\ldots,x_n]$, we compute the Hilbert series of the associated quotient rings via a simple numerical algorithm. We also conjecture the extension for $k>2$. Via Macaulay duality, those power ideals are related to schemes of fat points with support on the $k^n$ points $[1:\xi^{g_1}:\ldots:\xi^{g_n}]$ in $\mathbb{P}^n$. We compute Hilbert series, Betti numbers and Gr\"obner basis for such $0$-dimensional schemes. This explicitly determines the Hilbert series of the power ideal for all $k$: that this agrees with our conjecture for $k>2$ is supported by several computer experiments.