Rings Whose Indecomposable Modules Are Pure-Projective Or Pure-Injective Francois Couchot
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Arxiv:2005.02059V1 [Math.RA]
A new approach to Baer and dual Baer modules with some applications N. Ghaedan 1 and M.R. Vedadi 2 Department of Mathematical Sciences, Isfahan University of Technology, Isfahan, 84156-83111, IRAN. ABSTRACT. Let R be a ring. It is proved that an R-module M is Baer (resp. dual Baer) if and only if every exact sequence 0 → X → M → Y → 0 with Y ∈ Cog(MR) (resp. X ∈ Gen(MR)) splits. This shows that being (dual) Baer is a Morita invariant property. As more + applications, the Baer condition for the R-module M = HomZ(M, Q/Z) is investigated and shown that R is a von Neumann regular ring, if R+ is a Baer R-module. Baer modules with (weak) chain conditions are studied and determined when a Baer (resp. dual baer) module is a direct sum of mutually orthogonal prime (resp. co-prime) modules. While finitely generated dual Baer modules over commutative rings is shown to be semisimple, finitely generated Baer modules over commutative domain are studied. In particular, if R is commutative hereditary Noetherian domain then a finitely generated MR is Baer if and only if it is projective or semisim- ple. Over a right duo perfect ring, it is shown that every (dual) Baer modules is semisimple. Keywords: Baer module, character module, co-prime module, prime module, dual baer, regular ring, retractable module. MSC(2010): Primary: 16D10; 16D40 Secondary: 13C05; 13C10. 1. Introduction Throughout rings will have unit elements and modules will be right unitary. A ring R is said to be Baer if for every non-empty subset X of R, the right annihilator X in R is of the form eR for some e = e2 ∈ R. -
REPRESENTATION THEORY WEEK 9 1. Jordan-Hölder Theorem And
REPRESENTATION THEORY WEEK 9 1. Jordan-Holder¨ theorem and indecomposable modules Let M be a module satisfying ascending and descending chain conditions (ACC and DCC). In other words every increasing sequence submodules M1 ⊂ M2 ⊂ ... and any decreasing sequence M1 ⊃ M2 ⊃ ... are finite. Then it is easy to see that there exists a finite sequence M = M0 ⊃ M1 ⊃···⊃ Mk = 0 such that Mi/Mi+1 is a simple module. Such a sequence is called a Jordan-H¨older series. We say that two Jordan H¨older series M = M0 ⊃ M1 ⊃···⊃ Mk = 0, M = N0 ⊃ N1 ⊃···⊃ Nl = 0 ∼ are equivalent if k = l and for some permutation s Mi/Mi+1 = Ns(i)/Ns(i)+1. Theorem 1.1. Any two Jordan-H¨older series are equivalent. Proof. We will prove that if the statement is true for any submodule of M then it is true for M. (If M is simple, the statement is trivial.) If M1 = N1, then the ∼ statement is obvious. Otherwise, M1 + N1 = M, hence M/M1 = N1/ (M1 ∩ N1) and ∼ M/N1 = M1/ (M1 ∩ N1). Consider the series M = M0 ⊃ M1 ⊃ M1∩N1 ⊃ K1 ⊃···⊃ Ks = 0, M = N0 ⊃ N1 ⊃ N1∩M1 ⊃ K1 ⊃···⊃ Ks = 0. They are obviously equivalent, and by induction assumption the first series is equiv- alent to M = M0 ⊃ M1 ⊃ ··· ⊃ Mk = 0, and the second one is equivalent to M = N0 ⊃ N1 ⊃···⊃ Nl = {0}. Hence they are equivalent. Thus, we can define a length l (M) of a module M satisfying ACC and DCC, and if M is a proper submodule of N, then l (M) <l (N). -
Pure Injective Modules Relative to Torsion Theories
International Journal of Algebra, Vol. 8, 2014, no. 4, 187 - 194 HIKARI Ltd, www.m-hikari.com http://dx.doi.org/10.12988/ija.2014.429 Pure Injective Modules Relative to Torsion Theories Mehdi Sadik Abbas and Mohanad Farhan Hamid Department of Mathematics, University of Mustansiriya, Baghdad, Iraq Copyright c 2014 Mehdi Sadik Abbas and Mohanad Farhan Hamid. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Let τ be a hereditary torsion theory on the category Mod-R of right R-modules. A right R-module M is called pure τ-injective if it is injec- tive with respect to every pure exact sequence having a τ-torsion cok- ernel. Every module has a pure τ-injective envelope. A module M is called purely quasi τ-injective if it is fully invariant in its pure τ-injective envelope. A module M is called quasi pure τ-injective if maps from τ- dense pure submodules of M into M are extendable to endomorphisms of M. The class of pure τ-injective modules is properly contained in the class of purely quasi τ-injective modules which is in turn properly contained in the class of quasi pure τ-injectives. A torsion theoretic version of each of the concepts of regular and pure semisimple rings is characterized using the above generalizations of pure injectivity. Mathematics Subject Classification: 16D50 Keywords: (quasi) pure injective module, purely quasi injective module, torsion theory 1 Introduction We will denote by R an associative ring with a nonzero identity and by τ = (T ; F) a hereditary torsion theory on the category Mod-R of right R-modules. -
Modular Representation Theory of Finite Groups Jun.-Prof. Dr
Modular Representation Theory of Finite Groups Jun.-Prof. Dr. Caroline Lassueur TU Kaiserslautern Skript zur Vorlesung, WS 2020/21 (Vorlesung: 4SWS // Übungen: 2SWS) Version: 23. August 2021 Contents Foreword iii Conventions iv Chapter 1. Foundations of Representation Theory6 1 (Ir)Reducibility and (in)decomposability.............................6 2 Schur’s Lemma...........................................7 3 Composition series and the Jordan-Hölder Theorem......................8 4 The Jacobson radical and Nakayama’s Lemma......................... 10 Chapter5 2.Indecomposability The Structure of and Semisimple the Krull-Schmidt Algebras Theorem ...................... 1115 6 Semisimplicity of rings and modules............................... 15 7 The Artin-Wedderburn structure theorem............................ 18 Chapter8 3.Semisimple Representation algebras Theory and their of Finite simple Groups modules ........................ 2226 9 Linear representations of finite groups............................. 26 10 The group algebra and its modules............................... 29 11 Semisimplicity and Maschke’s Theorem............................. 33 Chapter12 4.Simple Operations modules on over Groups splitting and fields Modules............................... 3436 13 Tensors, Hom’s and duality.................................... 36 14 Fixed and cofixed points...................................... 39 Chapter15 5.Inflation, The Mackey restriction Formula and induction and Clifford................................ Theory 3945 16 Double cosets........................................... -
Cyclic Pure Submodules
International Journal of Algebra, Vol. 3, 2009, no. 3, 125 - 135 Cyclic Pure Submodules V. A. Hiremath1 Department of Mathematics, Karnatak University Dharwad-580003, India va hiremath@rediffmail.com Seema S. Gramopadhye2 Department of Mathematics, Karnatak University Dharwad-580003, India e-mail:[email protected] Abstract P.M.Cohn [3] has introduced the notion of purity for R-modules. With respect to purity, flat, absolutely pure and regular modules are studied. In this paper we introduce and study the corresponding no- tions of c-flat, absolutely c-pure and c-regular modules for cyclic pu- rity. We prove that absolutely c-pure R-modules are precisely injective modules. Also we study the relationship between c-flat and torsion-free modules over commutative integral domains and non-commutative non- integral domains. Also, we study the conditions under which c-regular R-modules are semi-simple. Mathematics Subject Classifications: 16D40, 16D50 Keywords: Pure submodules, flat module and regular ring Introduction In this paper, by a ring R we mean an associative ring with unity and by an R-module we mean a unitary right R-module. Z(M) denotes the singular submodule of the R-module M. 1Corresponding author 2The author is supported by University Research Scholarship. 126 V. A. Hiremath and S. S. Gramopadhye A ring R is said to be principal projective, if every principal right ideal is projective. We denote this ring by p.p. The notion of purity has an important role in module theory and in model theory. In model theory, the notion of pure exact sequence is more useful than split exact sequences. -
Block Theory with Modules
JOURNAL OF ALGEBRA 65, 225--233 (1980) Block Theory with Modules J. L. ALPERIN* AND DAVID W. BURRY* Department of Mathematics, University of Chicago, Chicago, Illinois 60637 and Department of Mathematics, Yale University, New Haven, Connecticut 06520 Communicated by Walter Felt Received October 2, 1979 1. INTRODUCTION There are three very different approaches to block theory: the "functional" method which deals with characters, Brauer characters, Carton invariants and the like; the ring-theoretic approach with heavy emphasis on idempotents; module-theoretic methods based on relatively projective modules and the Green correspondence. Each approach contributes greatly and no one of them is sufficient for all the results. Recently, a number of fundamental concepts and results have been formulated in module-theoretic terms. These include Nagao's theorem [3] which implies the Second Main Theorem, Green's description of the defect group in terms of vertices [4, 6] and a module description of the Brauer correspondence [1]. Our purpose here is to show how much of block theory can be done by statring with module-theoretic concepts; our contribution is the methods and not any new results, though we do hope these ideas can be applied elsewhere. We begin by defining defect groups in terms of vertices and so use Green's theorem to motivate the definition. This enables us to show quickly the fact that defect groups are Sylow intersections [4, 6] and can be characterized in terms of the vertices of the indecomposable modules in the block. We then define the Brauer correspondence in terms of modules and then prove part of the First Main Theorem, Nagao's theorem in full and results on blocks and normal subgroups. -
TORSION THEORIES OVER SEMIPERFECT RINGS (A) ¿Rnjs
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY Volume 34, Number 2, August 1972 TORSION THEORIES OVER SEMIPERFECT RINGS EDGAR A. RUTTER, JR. Abstract. In this note we characterize those torsion theories over a semiperfect ring such that the class of torsion free modules is closed under projective covers and the hereditary torsion theories for which this is true of the class of torsion modules. These results are applied to determine all hereditary torsion theories over an injective cogenerator ring with the property that the torsion sub- module of every module is a direct summand. The purpose of this note is to investigate some properties of torsion theories in the category of modules over a semiperfect ring. We charac- terize those torsion theories with the property that the class of torsion free modules is closed under projective covers and the hereditary torsion theories for which this is true of the class of torsion modules. A torsion theory is called splitting provided the torsion submodule of every module is a direct summand. Bernhardt [4] proved that every splitting hereditary torsion theory for a quasi-Frobenius ring is determined by a central idempotent of the ring. The results mentioned above permit us to extend this result to a natural generalization of quasi-Frobenius rings, namely, the one-sided injective cogenerator rings. Bernhardt's theorem depended on a result of Alin and Armendariz [1] which states that every hereditary torsion class over a right perfect ring is closed under direct products and hence is determined by an idempotent ideal of the ring. While we shall not require this result our methods yield a new proof of it. -
S-ALMOST PERFECT COMMUTATIVE RINGS Contents Introduction 1 1. Preliminaries 3 2. S-Divisible and S-Torsion Modules 5 3. S-Strong
S-ALMOST PERFECT COMMUTATIVE RINGS SILVANA BAZZONI AND LEONID POSITSELSKI Abstract. Given a multiplicative subset S in a commutative ring R, we consider S-weakly cotorsion and S-strongly flat R-modules, and show that all R-modules have S-strongly flat covers if and only if all flat R-modules are S-strongly flat. These equivalent conditions hold if and only if the localization RS is a perfect ring and, for every element s 2 S, the quotient ring R=sR is a perfect ring, too. The multiplicative subset S ⊂ R is allowed to contain zero-divisors. Contents Introduction 1 1. Preliminaries 3 2. S-divisible and S-torsion modules 5 3. S-strongly flat modules 7 4. S-h-local rings 9 5. t-contramodules 14 6. S-h-nil rings 16 7. S-almost perfect rings 22 8. The condition P1 = F1 25 References 28 Introduction Let R be a commutative ring and Q its total ring of quotients. An 1 R-module C is said to be weakly cotorsion if ExtR(Q; C) = 0. An R-module 1 F is strongly flat if ExtR(F; C) = 0 for all weakly cotorsion R-modules C. This definition first appeared in the paper [Trl01]. The problem of char- acterizing commutative domains R for which the class of all strongly flat modules is covering was posed in lecture notes [Trl00]. This problem was solved in the series of papers [BS04, BS02]. It was shown that, for a commutative domain R, the class of all strongly flat R-modules is covering if and only if it coincides with the class of all flat R-modules, and this holds if and only if all the quotient rings of R by nonzero ideals 2010 Mathematics Subject Classification. -
Pure Baer Injective Modules
Internat. J. Math. & Math. Sci. 529 VOL. 20 NO. 3 (1997) 529-538 PURE BAER INJECTIVE MODULES NADA M. AL THANI Department of Mathematics Faculty of Science Qatar University Doha, QATAR (Received April 19,1995 and in revised form April 16, 1996) ABSTRACT. In this paper we generalize the notion of pure injectivity of modules by introducing what we call a pure Baer injective module. Some properties and some characterization of such modules are established. We also introduce two notions closely related to pure Baer injectivity; namely, the notions of a E-pure Baer injective module and that of SSBI-ring. A ring R is an SSBI-ring if and only if every smisimple R-module is pure Baer injective. To investigate such algebraic structures we had to define what we call p-essential extension modules, pure relative complement submodules, left pure hereditary tings and some other related notions. The basic properties of these concepts and their interrelationships are explored, and are further related to the notions of pure split modules. KEY WORDS AND PHRASES: Pure and E-pure Baer injective modules, pure hereditary ring, pure- split module, P-essential extension submodule, pure relative complement submodule, SSBI-ring. 1991 AMS SUBJECT CLASSnZICATION CODES: 13C. 0. INTRODUCTION In this introduction we establish terminology and recall a summary of some basic definitions and results in the literature necessary for subsequent sections of the paper. Throughout R will denote a ring with identity. Unless otherwise stated, all modules will be unitary left R-modules, and all homomorphisms will be R-homomorphisms. A short exact sequence 0---, N---} M---} K--} 0 of left R-modules is said to be pure if" L (R) RN L (R) RM is a monomorphism for every right R-module L. -
On Harada Rings and Serial Artinian Rings
Vietnam Journal of Mathematics 36:2(2008) 229–238 Vietnam Journal of MATHEMATICS © VAST 2008 On Harada Rings and Serial Artinian Rings Thanakarn Soonthornkrachang1, Phan Dan2, Nguyen Van Sanh3, and Kar Ping Shum 4 1,3Department of Mathematics, Mahidol University, Bangkok 10400, Thailand 2Department of Mathematics, University of Transport of Ho Chi Minh City, Vietnam 4Department of Mathematics, The University of Hong Kong, Hong Kong, China (SAR) Received November 18, 2007 Abstract. A ring R is called a right Harada ring if it is right Artinian and every non-small right R-module contains a non-zero injective submodule. The first result in our paper is the following: Let R be a right perfect ring. Then R is a right Harada ring if and only if every cyclic module is a direct sum of an injective module and a small module; if and only if every local module is either injective or small. We also prove that a ring R is QF if and only if every cyclic module is a direct sum of a projective injective module and a small module; if and only if every local module is either projective injective or small. Finally, a right QF-3 right perfect ring R is serial Artinian if and only if every right ideal is a direct sum of a projective module and a singular uniserial module. 2000 Mathematics Subject Classification: 16D50, 16D70, 16D80. Keywords: Harada ring, Artinian Ring, Small module, Co-small module. 1. Introduction and Preliminaries Throughout this paper, all rings are associative rings with identity and all right R-modules are unitary. -
Decomposition of Graded Modules Cary Webb1
PROCEEDINGS of the AMERICAN MATHEMATICAL SOCIETY Volume 94, Number 4, August 1985 DECOMPOSITION OF GRADED MODULES CARY WEBB1 ABSTRACT. In this paper, the primary objective is to obtain decomposition theorems for graded modules over the polynomial ring k[x], where k denotes a field. There is some overlap with recent work of Hoppner and Lenzing. The results obtained include identification of the free, projective, and injective modules. It is proved that a module that is either reduced and locally finite or bounded below is a direct sum of cyclic submodules. Pure submodules are direct summands if they are bounded below. In such case, the pure submodule is itself a direct sum of cyclic submodules. It is also noted that Cohen and Gluck's Stacked Bases Theorem remains true if the modules are graded. I. Introduction. The purpose of this study is to obtain decomposition theo- rems for graded /c[i]-modules, where k denotes a field. Such a module is a direct sum, in the upgraded sense, of fc-vector spaces V¿ indexed over the positive and negative integers. The operation x on V¿ is an (arbitrary) linear transformation Xi'.Vi —» Vi+i. (Thus x actually represents a countable family of distinct linear transformations.) The V¿'s are called the homogeneous components of V. Our most important kind of results are those which decompose a graded k[x\- module into a direct sum of cyclic submodules and also those results which show how a submodule can be a direct summand. Important too but more easily derived are the identities of injective objects and the fact that all projective objects are free. -
On Rings Whose Flat Modules Form a Grothendieck Category
COLLOQUIUMMATHEMATICUM VOL. 73 1997 NO. 1 ON RINGS WHOSE FLAT MODULES FORM A GROTHENDIECK CATEGORY BY J . L . G A R C I A (MURCIA) AND D. S I M S O N (TORUN)´ 1. Introduction. Throughout this paper, by a ring we shall mean “a ring with enough idempotents” in the sense of [4] and [26, p. 464], that is, an associative ring R containing a set {eλ}λ∈Λ of pairwise orthogonal idempotent elements eλ, λ ∈ Λ, such that M M (1.1) R = Reλ = eλR. λ∈Λ λ∈Λ We say that the ring R is unitary if it has an identity element 1. In this case the set Λ is finite. By a right R-module we shall always mean a right R-module M which is unitary, that is, MR = M. We denote by Mod(R) the category of all unitary right R-modules, and thus Mod(Rop) will stand for the category of left R-modules. The full subcategory of Mod(R) formed by all finitely generated projective modules will be denoted by proj(R). A right R-module M is flat if the tensor product functor op M ⊗R (−) : Mod(R ) → Ab is exact. The full subcategory of Mod(R) consisting of all flat right R- modules will be denoted by Fl(R). For convenience, we introduce the fol- lowing definition. L L Definition 1.2. A ring R = λ∈Λ Reλ = λ∈Λ eλR as in (1.1) is right panoramic if the category Fl(R) of flat right R-modules is abelian, or, equivalently, if Fl(R) is a Grothendieck category.