Verdier quotients of homotopy categories of rings and Gorenstein-projective precovers

Let R be a ring, Proj be the class of all the projective right R-modules, K be the full subcategory of the homotopy category K(Proj) whose class of objects consists of all the totally acyclic complexes, and MorK be the class of all the morphisms in K(Proj) whose cones belong to K. We prove that if K(Proj) has enough MorK-injective objects, then the Verdier quotient K(Proj)/K has small Hom-sets, and this last condition implies the existence of Gorenstein-projective precovers in Mod-R and of totally acyclic precovers in C(Mod-R).

Gorenstein-Projective Modules over Morita Rings

Let△(φ,ψ)=(ABMA ANBB)be a Morita ring which is an Artin algebra.In this paper we investigate the relations between the Gorenstein-projective modules over a Morita ring△(φ,ψ)and the algebras A and B.We prove that if△(φ,ψ)is a Gorenstein algebra and both Ma and aN(resp.,both NB and BM)have finite projective dimension,then A(resp.,B)is a Gorenstein algebra.We also discuss when the CM-freeness and the CM-finiteness of a Morita ring△(φ,ψ)is inherited by the algebras A and B.

Gorenstein-Projective Modules over T_(m,n)(A)

A new class of Gorenstein algebras Tm,n(A) is introduced,their module categories are described,and all the Gorenstein-projective Tm,n(A)-modules are explicitly determined.

Morphism Categories of Gorenstein-projective Modules

Let A be an algebra of finite Cohen-Macaulay type and F its Cohen-Macaulay Auslander algebra. We are going to characterize the morphism category Mor（∧-Gproj） of Gorenstein-projective ∧-modules in terms of the module category F-mod by a categorical equivalence. Based on this, we obtain that some factor category of the epimorphism category Epi（∧-Gproj） is a Frobenius category, and also, we clarify the relations among Mor（∧-Gproj）, Mor（T2（∧）-Gproj） and Mor（△-Gproj）, where T2（∧） and △ are respectively the lower triangular matrix algebra and the Morita ring closely related to ∧.

Gorenstein projective modules over rings of Morita contexts

Under semi-weak and weak compatibility conditions of bimodules,we establish necessary and sufficient conditions of Gorenstein-projective modules over rings of Morita contexts with one bimodule homomorphism zero.This extends greatly the results on triangular matrix Artin algebras and on Artin algebras of Morita contexts with two bimodule homomorphisms zero in the literature,where only sufficient conditions are given under a strong assumption of compatibility of bimodules.An application is provided to describe Gorenstein-projective modules over noncommutative tensor products arising from Morita contexts.Our results are proved under a general setting of noetherian rings and modules instead of Artin algebras and modules.

The Short Local Algebras of Dimension 6 with Non-projective Reflexive Modules

Let A be a finite-dimensional local algebra over an algebraically closed field,let J be the radical of A.The modules we are interested in are the finitely generated left A-modules.Projectivemodules are always reflexive,and an algebra is self-injective iff allmodules are reflexive.We discuss the existence of non-projective reflexive modules in case A is not self-injective.We assume that A is short(this means that J^(3)=0).In a joint paper with Zhang Pu,it has been shown that 6 is the smallest possible dimension of A that can occur and that in this case the following conditions have to be satisfied:J^(2)is both the left socle and the right socle of A and there is no uniform ideal of length 3.The present paper is devoted to showing the converse.

Gorensteinness, homological invariants and Gorenstein derived categories

Relations between Corenstein derived categories, Gorenstein defect categories and Gorenstein stable categories are established. Using these, the Gorensteinness of an algebra A and invariants with respect to recolle- ments of the bounded Gorenstein derived category Dgbp（A-mod） of A are investigated. Specifically, the Goren- steinness of A is characterized in terms of recollements of Dbp（A-mod） and Corenstein derived equivalences. It is also shown that Cohen-Macaulay-finiteness is invariant with respect to the recollements of D^p（A-mod）.