Test ideals in diagonal hypersurface rings II
Abstract
Description
Let $R=k[x_1, ..., x_n]/(x_1^d + ... + x_n^d)$, where $k$ is a field of characteristic $p$, $p$ does not divide $d$ and $n \geq 3$. We describe a method for computing the test ideal for these diagonal hypersurface rings. This method involves using a characterization of test ideals in Gorenstein rings as well as developing a way to compute tight closures of certain ideals despite the lack of a general algorithm. In addition, we compute examples of test ideals in diagonal hypersurface rings of small characteristic (relative to $d$) including several that are not integrally closed. These examples provide a negative answer to Smith's (2000, Comm. in Alg.) question of whether the test id eal in general is always integrally closed.
revised version, incorporating referee's comments, LaTeX, 10 pages
revised version, incorporating referee's comments, LaTeX, 10 pages