Abstract. Dimer models are a combinatorial tool to describe certain algebras that appear as noncommutative crepant resolutions of toric Gorenstein singularities. Unfortunately, not every dimer model gives rise to a noncommutative crepant resolution. Several notions of consistency have been introduced to deal with this problem. In this paper, we study the major different notions in detail and show that for dimer models on a torus, they are all equivalent.2010 Mathematics Subject Classification. 14M25, 14A22, 16S38.
Introduction.If X is a three-dimensional normal Gorenstein singularity admitting a crepant resolutionX → X, then one is interested to describe the bounded derived category DCohX of coherent sheaves onX. A well-known result by Bridgeland [4] shows that this category only depends on the singularity and not on the particular choice of crepant resolution.In many cases, there exists a tilting bundle in U ∈ DCohX such that DCohX is equivalent as a triangulated category to the derived category of finitely generated A-modules DModA, where A = EndX . To model these algebras without referring to a commutative crepant resolution, Van den Bergh [23] introduced the notion of a noncommutative crepant resolution (NCCR) of X. This is a homologically homogeneous algebra of the form A = End R (T), where T is a reflexive R-module, with R = [ރX] the coordinate ring of the singularity. An NCCR is, however, far from unique and in general there are an infinite number of different noncommutative crepant resolutions.If we make two restrictions, the problem becomes more manageable. First, we assume that X is a toric three-dimensional Gorenstein singularity. This automaticly implies the existence of a commutative crepant resolution. Secondly, we assume that the tilting bundle is a direct sum of nonisomorphic line bundles. It was noticed in string theory [8,10,11] that, under these conditions, the algebra A can be described using a dimer model on a torus.This means that A is the path algebra of a quiver Q with relations where Q is embedded in a two-dimensional real torus T such that every connected piece of T \ Q is bounded by a cyclic path of length at least 3. The relations are given by demanding for every arrow a that p = q where ap and aq are the bounding cycles that contain a.This nice description follows from the fact that the algebra A is a toric order, a special type of order compatible with the toric structure, and Calabi-Yau-3 (CY-3), i.e. it admits a self-dual bimodule resolution of length 3. In [2], it was shown that every toric CY-3 order comes from a dimer model. Not every dimer model gives a noncommutative crepant resolution of its centre. To do so, it needs to satisfy some extra conditions called consistency conditions. In recent years,