Abstract. Let R be a commutative Noetherian local ring. This paper deals with the problem asking whether R is Gorenstein if the nth syzygy module of the residue class field of R has a non-trivial direct summand of finite Gdimension for some n. It is proved that if n is at most two then it is true, and moreover, the structure of the ring R is determined essentially uniquely.
IntroductionThroughout the present paper, we assume that all rings are commutative Noetherian local rings and all modules are finitely generated modules.G-dimension is a homological invariant of a module which has been introduced by Auslander [1]. This invariant is an analogue of projective dimension. Whereas the finiteness of projective dimension characterizes the regular property of the base ring, the finiteness of G-dimension characterizes the Gorenstein property of the base ring. To be precise, any module over a Gorenstein local ring has finite G-dimension, and a local ring with residue class field of finite G-dimension is Gorenstein. Gdimension shares a lot of properties with projective dimension. For example, it also satisfies an Auslander-Buchsbaum-type equality, which is called the AuslanderBridger formula.Dutta [9] proved the following theorem in his research into the homological conjectures: Theorem 1.0.1 (Dutta). Let (R, m, k) be a local ring. Suppose that the nth syzygy module of k has a non-zero direct summand of finite projective dimension for some n ≥ 0. Then R is regular.Since G-dimension is similar to projective dimension, this theorem naturally leads us to the following question: Question 1.0.2. Let (R, m, k) be a local ring. Suppose that the nth syzygy module of k has a non-zero direct summand of finite G-dimension for some n ≥ 0. Then is R Gorenstein?It is obviously seen from the indecomposability of k that this question is true if n = 0. Hence this question is worth considering just in the case where n ≥ 1.We are able to answer in this paper that the above question is true if n ≤ 2. Furthermore, as the theorems below say, we can even determine the structure of a ring satisfying the assumption of the above question for n = 1, 2.2000 Mathematics Subject Classification. Primary 13D02; Secondary 13D05, 13H10.