A study of a quadratic almost complete intersection ideal and its linked Gorenstein ideal

Abstract

We examine the ideal $I=(x_1^2,\dots ,x_n^2,(x_1+\cdots +x_n{)}^2)$ in the polynomial ring $Q=k[x_1,\dots ,x_n]$, where $k$ is a field of characteristic zero or greater than $n$. We also study the Gorenstein ideal $G$ linked to $I$ via the complete intersection ideal $(x_1^2,\dots ,x_n^2)$. We compute the Betti numbers of $I$ and $G$ over $Q$ when $n$ is odd and extend known computations when $n$ is even. A consequence is that the socle of $Q/I$ is generated in a single degree (thus $Q/I$ is level) and its dimension is a Catalan number. We also describe the generators and the initial ideal with respect to reverse lexicographic order for the Gorenstein ideal $G$.