arXiv subjects

Michael Hellus

Publications and source records attributed to Michael Hellus.

25 records · Page 2Linked to original sources

arXiv2007

A Note on the injective dimension of local cohomology modules

For a noetherian ring R we call an R-module M cofinite if there exists an ideal I of R such that M is I-cofinite; we show that every cofinite module M satisfies dim_R(M)<=injdimR(M). As an application we study the question which local cohomology modules H^i_I(R) satisfy injdim_R(H^i_I(R)) = dim_R(H^i_I(R)). There are two situations where the answer is positive. On the other hand we present two counter-examples, the failure in these two examples coming from different reasons.

math.AC↗

arXiv2006

On the set of associated primes of a local cohomology module

Assume $R$ is a local Cohen-Macaulay ring. It is shown that $\Ass_R (H^l_I(R))$ is finite for any ideal $I$ and any integer $l$ provided $\Ass_R (H^2_{(x,y)}(R))$ is finite for any $x,y\in R$ and $\Ass_R (H^3_{(x_1,x_2,y)}(R))$ is finite for any $y\in R$ and any regular sequence $x_1,x_2\in R$. Furthermore it is shown that $\Ass_R (H^l_I(R))$ is always finite if $\dim (R)\leq 3$. The same statement is even true for $\dim (R)\leq 4$ if $R$ is almost factorial.

math.AC↗

arXiv2006

Finiteness properties of duals of local cohomology modules

We investigate Matlis duals of local cohomology modules and prove that, in general, their zeroth Bass number with respect to the zero ideal is not finite. We also prove that, somewhat surprisingly, if we apply local cohomology again (i. e. to the Matlis dual of the local cohomology module), we get (under certain hypotheses) either zero or $E$, an $R$-injective hull of the residue field of the local ring $R$.

math.AC↗

arXiv2006

Attached primes and Matlis duals of local cohomology modules

Let $J$ be an ideal of a noetherian local ring $R$. We show new results on the set of attached primes $\Att_R(\LCMo ^l_J(R))$ of a local cohomology module $\LCMo ^l_J(R)$. To prove our results we establish and use new relations between the set $\Att_R(\LCMo ^l_J(R))$ of attached primes of a local cohomology module and the set $\Ass_R(D(\LCMo ^l_J(R)))$ of associated primes of the Matlis dual of the same local cohomology module.

math.AC↗

arXiv2005

Local Cohomology and complete intersections of rank 1

The number of equations needed to cut out a variety given by an ideal is called the arithmetic rank (of the ideal). It was shown in [8] that the notion of arithmetic rank is strongly related to the concept of regular sequences on the Matlis duals of certain local cohomology modules and thus is related to the set of associated primes of such Matlis duals. While in [8] the general situation with no restrictions on arithmetic rank and cohomological dimension was investigated, this work concentrates on the case ``cohomological dimension of the given ideal is (at most) one'' and also on the interplay between non-graded local and graded situations. The main results are new characterizations for the case $\ara \leq 1$ (theorem 1 and theorem 2), the fact that given a graded ring $R$ and a homogenous ideal $I$ of cohomological dimension at most one the inclusion $$\{x\in I| x\hbox {non-homogenous}\} \subseteq \bigcup_{\goth p\in \Ass_R(D(\LCMo ^1_I(R)))}\goth p$$ holds (theorem 4, $D$ is a Matlis dual functor) and a (somewhat technical) statement on regular sequences on Matlis duals of certain local cohomology modules (theorem 5), which shows that regular sequences on such Matlis duals behave well in some sense although these modules are not finite in general.

math.AC↗

arXiv2005

Local Homology, Cohen-Macaulayness and Cohen-Macaulayfications

Let (R,m) be a local, complete ring, X an artinian R-module of Noetherian dimension d; let x_1,...,x_d\in m be such that 0:_X (x_1,...,x_d)R has finite length. Then H^x_d(X) is a finite R-module, providing a positive answer to a question posed by Tang. As a first application of this result corollory 1 contains a necessary condition for a finite module to be CM; secondly we propose a notion of Cohen-Macaulayfication and prove its uniqueness (th. 3); finally we show that this new notion of Cohen-Macaulayfication is a direct generalization of a notion of Cohen-Macaulayfication introduced by Goto (th. 4).

math.AC↗

arXiv2004

On the associated primes of Matlis duals of top local cohomology modules

After motivating the question we prove various results about the set of associated primes of Matlis duals of top local cohomology modules. In some cases we can calculate this set, for the general situation we present a conjecture. An easy application of this theory is the well-known fact that Krull dimension can be expressed by the vanishing of local cohomology modules.

math.AC↗