Toric Rings, Inseparability and Rigidity

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 $m-k\ge k\ge 4$, $G_{k,m-k}$ is rigid. Here $G_{k,m-k}$ is the complete bipartite graph $K_{m-k,k}$ with one edge removed.

Part of this article was written while the third author was visiting the Department of Mathematics of University Duisburg-Essen. He wants to express his thanks for the hospitality.