Toric rings, inseparability and rigidity

TL;DR

This paper studies infinitesimal deformations of toric rings, providing explicit descriptions of their deformation spaces, and explores properties like inseparability and rigidity, with applications to graph edge rings and convex polyominoes.

Contribution

It offers an explicit description of the cotangent module for toric rings, introduces the concepts of inseparability and semi-rigidity, and applies these to graph edge rings and convex polyominoes.

Findings

Convex polyomino coordinate rings are inseparable.

Complete bipartite graphs with one edge removed are rigid for certain parameters.

Introduces semi-rigidity and characterizes graphs with semi-rigid edge rings.

Abstract

This article provides the basic algebraic background on infinitesimal deformations and presents the proof of the well-known fact that the non-trivial infinitesimal deformations of a $K$-algebra $R$ are parameterized by the elements of cotangent module $T^1(R)$ of $R$. In this article we focus on deformations of toric rings, and give an explicit description of $T^1(R)$ in the case that $R$ is a toric ring. In particular, we are interested in unobstructed deformations which preserve the toric structure. Such deformations we call separations. Toric rings which do not admit any separation are called inseparable. We apply the theory to the edge ring of a finite graph. The coordinate ring of a convex polyomino may be viewed as the edge ring of a special class of bipartite graphs. It is shown that the coordinate ring of any convex polyomino is inseparable. We introduce the concept of semi-rigidity, and give a combinatorial description of the graphs whose edge ring is semi-rigid. The results are applied to show that for $m-k=k=3$, $G_{k,m-k}$ is not rigid while for $m-k\geq k\geq 4$, $G_{k,m-k}$ is rigid. Here $G_{k,m-k}$ is the complete bipartite graph $K_{m-k,k}$ with one edge removed.