Vanishing of cohomology over Cohen--Macaulay rings

A 2003 counterexample to a conjecture of Auslander brought attention to a family of rings - colloquially called AC rings - that satisfy a natural condition on vanishing of cohomology. Several results attest to the remarkable homological properties of AC rings, but their definition is barely operational, and it remains unknown if they form a class that is closed under typical constructions in ring theory. In this paper, we study transfer of the AC property along local homomorphisms of Cohen--Macaulay rings. In particular, we show that the AC property is preserved by standard procedures in local algebra. Our results also yield new examples of Cohen-Macaulay AC rings.Comment: Updated references. Final version to appear in Manuscripta Math.; 9 p

Gorenstein dimension of modules over homomorphisms

Given a homomorphism of commutative noetherian rings R --> S and an S-module N, it is proved that the Gorenstein flat dimension of N over R, when finite, may be computed locally over S. When, in addition, the homomorphism is local and N is finitely generated over S, the Gorenstein flat dimension equals sup{m | Tor^R_m(E,N) \noteq 0} where E is the injective hull of the residue field of R. This result is analogous to a theorem of Andr\'e on flat dimension.Comment: 14 pp. To appear in J. Pure Appl. Algebra. Also available from http://www.math.unl.edu/~lchristensen3/index.htm

Ascent Properties of Auslander Categories

Let R be a homomorphic image of a Gorenstein local ring. Recent work has shown that there is a bridge between Auslander categories and modules of finite Gorenstein homological dimensions over R. We use Gorenstein dimensions to prove new results about Auslander categories and vice versa. For example, we establish base change relations between the Auslander categories of the source and target rings in a homomorphism R -> S of finite flat dimension.Comment: Minor corrections; example added; 30 pp. To appear in Canad. J. Math. Also available from authors' homepages http://www.math.unl.edu/~lchristensen3/publications.html and http://home.imf.au.dk/holm/publications.htm

Descent via Koszul extensions

Let R be a commutative noetherian local ring with completion R^. We apply differential graded (DG) algebra techniques to study descent of modules and complexes from R^ to R' where R' is either the henselization of R or a pointed \'etale neighborhood of R: We extend a given R^-complex to a DG module over a Koszul complex; we describe this DG module equationally and apply Artin approximation to descend it to R. This descent result for Koszul extensions has several applications. When R is excellent, we use it to descend the dualizing complex from R^ to a pointed \'etale neighborhood of R; this yields a new version of P. Roberts' theorem on uniform annihilation of homology modules of perfect complexes. As another application we prove that the Auslander Condition on uniform vanishing of cohomology ascends to R^ when R is excellent, henselian, and Cohen--Macaulay.Comment: Updated references and made minor changes. Final version, to appear in J. Algebra; 19 p