Conic divisorial ideals and non-commutative crepant resolutions of edge rings of complete multipartite graphs

Abstract

The first goal of the present paper is to study the class groups of the edge rings of complete multipartite graphs, denoted by $\mathbb{k}[K_{r_1,\dots ,r_n}]$, where $1\le r_1\le \cdots \le r_n$. More concretely, we prove that the class group of $\mathbb{k}[K_{r_1,\dots ,r_n}]$ is isomorphic to ${\mathbb{Z}}^n$ if $n=3$ with $r_1\ge 2$ or $n\ge 4$, while it turns out that the excluded cases can be deduced into Hibi rings. The second goal is to investigate the special class of divisorial ideals of $\mathbb{k}[K_{r_1,\dots ,r_n}]$, called conic divisorial ideals. We describe conic divisorial ideals for certain $K_{r_1,\dots ,r_n}$ including all cases where $\mathbb{k}[K_{r_1,\dots ,r_n}]$ is Gorenstein. Finally, we give a non-commutative crepant resolution (NCCR) of $\mathbb{k}[K_{r_1,\dots ,r_n}]$ in the case where it is Gorenstein.

21 pages