
doi: 10.1007/bf01196355
Let A be a commutative Noetherian ring with identity. Following the author [J. Lond. Math. Soc., II. Ser. 19, 402-410 (1979; Zbl 0396.13016)] a non-empty set of ideals of A, say \(\Phi\), is called a system of ideals of whenever \({\mathfrak a},{\mathfrak b}\in \Phi\), then there is an ideal \({\mathfrak c}\) in \(\Phi\) such that \({\mathfrak c}\subseteq {\mathfrak a}\cdot {\mathfrak b}\). For such a system, for every A-module M we define \(L_{\Phi}(M)=\{m\in M| \quad {\mathfrak a}\cdot m=0\) for some \({\mathfrak a}\in \Phi\}.\) This functor is called the general local cohomology functor with respect to (w.r.t.) \(\Phi\). For any A-module M, the modules \(H^ i_{\Phi}(M)\) are called general local cohomology modules of M, where \(H^ i_{\Phi}\) is the i-th right derived functor of \(L_{\Phi}\). Also following \textit{R. Y. Sharp} and \textit{H. Zakeri} [Mathematika 29, 32-41 (1982; Zbl 0497.13006)], let \(D_ n(A)\) denote the set of all \(n\times n\) lower triangular matrices over A. A triangular subset of \(A^ n\) is a non- empty subset U of \(A^ n\) such that (i) whenever \((u_ 1,...,u_ n)\in U\), then \((u_ 1^{\alpha _ 1},...,u_ n^{\alpha _ n})\in U\) for all choices of positive integers \(\alpha _ 1,...,\alpha _ n\), and (ii) whenever \((u_ 1,...,u_ n),(v_ 1,...,v_ n)\in U\), then there exists an element \((w_ 1,...,w_ n)\in U\) and matrices \(H,K\in D_ n(A)\) such that \(H[u_ 1,...,u_ n]^ T=[w_ 1,...,w_ n]^ T=K[v_ 1,...,v_ n]^ T\) where T means transpose operation on matrices. Now let U be a triangular subset of \(A^ n\) and M an A-module. The module of generalized fractions of M w.r.t. U, which is denoted by \(U^{-n}M\), is the set of elements of the form \(m/(u_ 1,..,u_ n)\) for \(m\in M\) and \((u_ 1,...,u_ n)\in U\), in which \(m/(u_ 1,...,u_ n)\) denotes the equivalence class of \((m,(u_ 1,...,u_ n))\in M\times U\) under the equivalence relation \(\sim\) defined as follows. For elements \((a,(u_ 1,...,u_ n))\) and \((b,(v_ 1,...,v_ n))\) of \(M\times U\) we write \((a,(u_ 1,...,u_ n))\sim (b,(v_ 1,...,v_ n))\) if and only if there exists \((w_ 1,...,w_ n)\in U\) and \(H,K\in D_ n(A)\) such that \(H[u_ 1,...,u_ n]^ T=[w_ 1,...,w_ n]^ T=K[v_ 1,...,v_ n]^ T\) and \(| H| a-| K| b\in (\sum ^{n- 1}_{i=1}Aw_ i)M.\) The module operations for \(U^{-n}M\) are as follows. If a,b\(\in M\) and \((u_ 1,...,u_ n),(v_ 1,...,v_ n)\in U\). then \(a/(u_ 1,...,u_ n)+b/(v_ 1,...,v_ n)=(Ha+Kb)/(w_ 1,...,w_ n)\) for any choice of \((w_ 1,...,w_ n)\in U\) and \(H,K\in D_ n(A)\) for which \(Hu=Kv\). (Here for any element \((s_ 1,...,s_ n)\in U\), the matrix \([s_ 1,...,sn]^ T\) is denoted by s.) Also for \(\lambda\) \(\in A:\) \(\lambda (a/(u_ 1,...,u_ n))=\lambda a/(u_ 1,...,u_ n)\). If U is a triangular subset of \(A^ n\), the set \(U[1]=\{(u_ 1,...,u_ n,1)| (u_ 1,...,u_ n)\in U\}\) is a triangular subset of \(A^{n+1}\) also. Now suppose A is a Noetherian local ring of dimension n. Then \(U=\{(u_ 1,...,u_ n)| u_ 1,...,u_ n\) is a system of parameters for \(A\}\) is a triangular subset of \(A^ n\). - It is proved [\textit{R. Y. Sharp} and \textit{H. Zakeri}, Mathematika 29, 296-306 (1982; Zbl 0523.13001)] that \(U[1]^{-n-1}M\cong H^ n_{{\mathfrak m}}(M)\) where \(H^ n_{{\mathfrak m}}(M)\) is the n-th local cohomology module of M w.r.t. the unique maximal ideal \({\mathfrak m}\) of A, \(n=\dim A\), and U is the above triangular subset of \(A^ n\) consisting of systems of parameters. Then as a generalization of this, let \({\mathfrak a}\) be an ideal of A which is generated by \(x_ 1,...,x_ n\), and \(U_ x\) be the triangular subset \(U_ x=\{x_ 1^{\alpha _ 1},...,x_ n^{\alpha _ n})| \quad \alpha _ 1,...,\alpha _ n\in {\mathbb{N}}\}\) of \(A^ n\). Then [see \textit{G. J. Gibson}, J. Lond. Math. Soc., II. Ser. 33, 245-252 (1986; Zbl 0562.13009)] \(U_ x[1]^{-n-1}M\cong H^ n_{{\mathfrak a}}(M).\) In this paper is proved a further generalization of these results, which is in terms of general local cohomology modules: Let U be an triangular subset of \(A^ n\). Take \(\Phi\) as the set of all ideals which are generated by an element of U. Then \(\Phi\) is a system of ideals of A and moreover we have the isomorphism \(U[1]^{-n-1}M\cong H^ n_{\Phi}(M)\). Moreover any system of ideals defines a torsion theory over A. This is the theory with the filter D generated by \(\Phi\) as follows \(D=\{{\mathfrak b},\) an ideal of \(A| \quad {\mathfrak b}\supseteq {\mathfrak a}\) for some \({\mathfrak a}\in \Phi \}.\) Thus any triangular subset U of \(A^ n\) defines a torsion theory \({\mathcal F}\) over A in a natural way. At the end, as an application, it is proved that if \({\mathcal F}\) is a torsion theory in which its idempotent filter has a cofinal subset of n- generated ideals (i.e. ideals generated by n elements), and that T is the corresponding torsion radical, then for any A-modules M and N\ \(T_ n(M\otimes _ AN)\simeq T_ n(M)\otimes _ AN\) where \(T_ n\) is the n-th right derived functor of T.
(Co)homology of commutative rings and algebras (e.g., Hochschild, André-Quillen, cyclic, dihedral, etc.), system of ideals, general local cohomology modules, triangular subset, module of generalized fractions, Rings of fractions and localization for commutative rings, Torsion modules and ideals in commutative rings, local ring, torsion theory, system of parameters
(Co)homology of commutative rings and algebras (e.g., Hochschild, André-Quillen, cyclic, dihedral, etc.), system of ideals, general local cohomology modules, triangular subset, module of generalized fractions, Rings of fractions and localization for commutative rings, Torsion modules and ideals in commutative rings, local ring, torsion theory, system of parameters
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