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On Graded Local Cohomology Modules Defined by a Pair of Ideals

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On Graded Local Cohomology Modules Defined by a Pair of Ideals Journal of Algebraic Systems Vol. 3, No. 2, (2016), pp 133-146 ON GRADED LOCAL COHOMOLOGY MODULES DEFINED BY A PAIR OF IDEALS M. JAHANGIRI∗ AND Z. HABIBI L Abstract. Let R = R be a standard graded ring, M be n2N0 n a finitely-generated graded R-module, and J be a homogeneous ideal of R. In this work, we study the graded structure of the i-th local cohomology module of M, defined by a pair of ideals (R+;J), i.e. Hi (M). More precisely, we discuss the finiteness property R+;J and vanishing of the graded components Hi (M) . Also, we R+;J n study the Artinian property and tameness of certain submodules and quotient modules of Hi (M). R+;J 1. Introduction Let R denotes a commutative Noetherian ring, M be an R-module, and I and J stand for two ideals of R. Takahashi et al. in [10], introduced the i-th local cohomology functor with respect to (I;J), i − denoted by HI;J ( ), as the i-th right derived functor of the (I;J)- Archive− off 2 SIDn ⊆ torsion functor ΓI;J ( ), where ΓI;J (M) := x M : I x Jx for n ≫ 1g. This notion is the ordinary local cohomology functor when J = 0 (see [3]). The main motivation for this generalization comes from MSC(2010): Primary: 13D45; Secondary: 13A02, 13E10. Keywords: Graded modules, Local cohomology module with respect to a pair of ideals, Artinian modules, Tameness. Received: 14 April 2015, Revised: 28 December 2015. ∗Corresponding author. 133 www.SID.ir 134 JAHANGIRI AND HABIBI i the study of a dual of ordinary local cohomology modules HI (M) (see [9]). Basic facts and more information about local cohomology defined by a pair of ideals can be obtained from [4], [5], and [10]. ⊕ Now let R = n2N0 Rn be a standard graded Noetherian ring, i.e. the base ring R0 is a commutative Noetherian ring, and R is generated, as an R0-algebra, by finitely many elements of R1. Also, let J be a homogeneous ideal of R, M be a graded R-module, and R+ := ⊕n2NRn be the irrelevant ideal of R. It is well-known ([3, Section 12]) that for ≥ i all i 0, the i-th local cohomology module HJ (M) of M with respect to J has a natural grading, and that in the case where M is finitely i 2 Z generated, HR+ (M)n is a finitely generated R0-module for all n , and it vanishes for all n ≫ 0 ([3, Theorem 15.1.5]). i In this paper, first, we show that HI;J (M) has a natural grading, when I and J are homogeneous ideals of R, and M is a graded R- module. Then, we show that, although in spite of the ordinary case, i 2 Z HR+;J (M)n might be non-finitely generated over R0 for some n and non-zero for all n ≫ 0, in some special cases, they are finitely- generated for all n 2 Z and vanish for all n ≫ 0. More precisely, we show that if (R0; m0) is local, R+ ⊆ b is an ideal of R, J0 is an ideal of T1 R , and kHi (M) = 0 for all n ≫ 0, then Hi (M) = 0 for 0 m0 b;J0R n b;J0R n k=0 all n ≫ 0 (Theorem 3.2). Also, we present an equivalent condition for i the finiteness of components HR+;J (M)n (Theorem 3.3). In the last section, first, we study the asymptotic stability of the set i f g 2Z ! −∞ AssR0 (HR+;J (M)n) n for n in a special case (Theorem 4.1). Then we present some results about Artinianness of some quotients i of HR Archive;J (M). In particular, we show thatof if R 0 SIDis a local ring with + 2 Z i maximal ideal m0 and c such that HR+;m0R(M) is Artinian for c c all i > c, then the R-module HR+;m0R(M)=m0HR+;m0R(M) is Artinian i (Theorem 4.2). Finally, we show that HR+;J (M) is "tame" in a special case (Corollary 4.4). www.SID.ir ON GRADED LOCAL COHOMOLOGY MODULES ... 135 2. graded local cohomology modules defined by a pair of ideals Throughout this section, let R = ⊕n2ZRn be a graded ring, I and J be two homogeneous ideals of R and M = ⊕n2ZMn be a graded R-module. Then it is natural to ask whether the local cohomology i 2 N modules HI;J (M) for all i 0, also carry structures as graded R- modules. In this section, we show that it has an affirmative answer. First we show that the (I;J)-torsion functor ΓI;J (−) can be viewed as a (left exact, additive) functor from ∗C to itself. Since the category ∗C of all graded R-modules and homogeneous R-homomorphisms, is an Abelian category that has enough projective objects and enough injective objects, we can, therefore, carry out standard techniques of homological algebra in this category. Hence, we can form the right ∗ i − − ∗C derived functors HI;J ( ) of ΓI;J ( ) on the category . ··· 2 2 Lemma 2.1. Let x = xi1 + + xik M be such that xij Mij for all j = 1; ··· ; k. Then: k r(Ann (x)) = \ r(Ann (xi )): j=1 j Proof. ⊇: Is clear. ⊆ ··· 2 : First we show that if y = yl1 + + ylm Ann (x) such that 2 ··· ··· 2 ylk Rlk for all k = 1; ; m and l1 < l2 < < lm, then yl1 \k j=1 r(Ann (xij )). We have: Xn Xm ∗ 0 = yx = ylk xij ; ( ) Archivej=1 k=1 of SID comparing degrees, we get yl1 xi1 = 0. Let j0 > 1, and suppose, induc- 0 j0 tively, that for all j < j , y x 0 = 0. Then using (∗), we get: 0 l1 ij Xn Xm − y yj0 1x = 0: lk l1 ij j=1 k=1 Again, comparing degrees, we have: yj0 x = 0. Thus y 2 \k r(Ann (x )). l1 ij0 l1 j=1 ij www.SID.ir 136 JAHANGIRI AND HABIBI ··· 2 Now let y = yl1 + + ylm r(Ann (x)) such that l1 < l2 < : : : < lm 2 ··· 2 N and ylj Rlj for all j = 1; ; m. Then, there exists s 0 such that ysx = 0. 2 \k In order to show that y j=1 r(Ann (xij )), we proceed by induc- tion on m. The result is clear in the case of m = 1. Now sup- pose, inductively, that m > 1, and the result has been proved for all values less than m. Using the above agreement, we know that k yl 2 \ r(Ann (xi )) ⊆ r(Ann (x)). Then yl + ··· + yl = y − yl 2 1 j=1 j 2 m 1 k r(Ann (x)). By inductive hypothesis, yl + ··· + yl 2 \ r(Ann (xi )), 2 m j=1 j k and so y 2 \ r(Ann (xi )). □ j=1 j Lemma 2.2. ΓI;J (M) is a graded R-module. 2 ··· Proof. Let x ΓI;J (M). Assume that x = xi1 + + xik , where, for ··· 2 ··· all j = 1; 2; ; k, xij Mij and i1 < i2 < < ik. We show that ··· 2 0 2 N xi1 ; ; xik ΓI;J (M). Since R is Noetherian, there is t such that t0 ⊆ ··· 2 N (r(Ann (xij ))) Ann (xij ) for all j = 1; 2; ; k. Let n 0 be such that In ⊆ Ann (x) + J. Thus by Lemma 2.1, for all j = 1; 2; ··· ; k, we have: 0 0 0 0 0 0 I2nt ⊆ (Ann (x) + J)2t ⊆ (Ann (x))t + J t ⊆ (r(Ann (x)))t + J t \k t0 t0 ⊆ = ( j=1r(Ann (xij ))) + J Ann (xij ) + J: 2 □ Thus xij ΓI;J (M), as required. ∗ i 2 N To calculate the graded local cohomology module HI;J (M)(i 0), one proceeds as follows: TakingArchive an ∗injective resolution, of SID 0 1 i E• : 0 ! E0 −!d E1 −!d E2 !···! Ei −!d Ei+1 !··· ; ∗ of M in C, applying the functor ΓI;J (−) to it, and taking the i-th cohomology module of this complex, we get: i kerΓI;J (d ) i−1 ; imΓI;J (d ) ∗ i which is denoted by HI;J (M), and is a graded R-module. www.SID.ir ON GRADED LOCAL COHOMOLOGY MODULES ... 137 f g Remark 2.3. Let 0 ! L −! M −! N ! 0 be an exact sequence of R-modules and R-homomorphisms. Then, for each i 2 N0, there is a ∗ i ! ∗ i+1 homogeneous connecting homomorphism, HI;J (N) HI;J (L), and these connecting homomorphisms make the resulting homogeneous long exact sequence: ∗H0 (f) ∗H0 (g) ! ∗ 0 −−−−−!I;J ∗ 0 −−−−−!I;J ∗ 0 0 HI;J (L) HI;J (M) HI;J (N) ∗H1 (f) ∗H1 (g) ! ∗ 1 −−−−−!I;J ∗ 1 −−−−−!I;J ∗ 1 HI;J (L) HI;J (M) HI;J (N) !··· ∗Hi (f) ∗Hi (g) ! ∗ i −−−−−!I;J ∗ i −−−−−!I;J ∗ i HI;J (L) HI;J (M) HI;J (N) ! ∗ i+1 !··· HI;J (L) : The reader should also be aware of the 'natural' or 'functorial' prop- erties of these long exact sequences. Definition 2.4. We define a partial order on the set: ∗Wf(I;J) := f a : a is a homogeneous ideal of R; In ⊆ a + J for some integer n ≥ 1g; by letting a ≤ b if a ⊇ b for a; b 2 ∗Wf(I;J). If a ≤ b, then we ∗f have Γa(M) ⊆ Γb(M). Therefore, the relation ≤ on W (I;J) with to- f g gether the inclusion maps make Γa(M) a2 ∗Wf(I;J) into a direct system of graded R-modules.
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