Ulrich Split Rings

A local Cohen–Macaulay ring is called Ulrich split if all short exact sequences of Ulrich modules split. In this paper we initiate the study of Ulrich split rings. We prove several necessary or sufficient criteria for this property, linking it to syzygies of the residue field and cohomology annihilator. We characterize Ulrich split rings of small dimensions. Over complex numbers, 2-dimensional Ulrich split rings, which are normal and have minimal multiplicity, are precisely cyclic quotient singularities with at most two indecomposable Ulrich modules up to isomorphism. We give several ways to construct Ulrich split rings and give a range of applications, from studying extremal behavior of test ideal of module closure operation associated to the family of maximal Cohen–Macaulay modules, to detecting projective/injective modules via vanishing of Ext.