Lectures on Commutative Algebra
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Dimension Theory and Systems of Parameters
Dimension theory and systems of parameters Krull's principal ideal theorem Our next objective is to study dimension theory in Noetherian rings. There was initially amazement that the results that follow hold in an arbitrary Noetherian ring. Theorem (Krull's principal ideal theorem). Let R be a Noetherian ring, x 2 R, and P a minimal prime of xR. Then the height of P ≤ 1. Before giving the proof, we want to state a consequence that appears much more general. The following result is also frequently referred to as Krull's principal ideal theorem, even though no principal ideals are present. But the heart of the proof is the case n = 1, which is the principal ideal theorem. This result is sometimes called Krull's height theorem. It follows by induction from the principal ideal theorem, although the induction is not quite straightforward, and the converse also needs a result on prime avoidance. Theorem (Krull's principal ideal theorem, strong version, alias Krull's height theorem). Let R be a Noetherian ring and P a minimal prime ideal of an ideal generated by n elements. Then the height of P is at most n. Conversely, if P has height n then it is a minimal prime of an ideal generated by n elements. That is, the height of a prime P is the same as the least number of generators of an ideal I ⊆ P of which P is a minimal prime. In particular, the height of every prime ideal P is at most the number of generators of P , and is therefore finite. -
3. Some Commutative Algebra Definition 3.1. Let R Be a Ring. We
3. Some commutative algebra Definition 3.1. Let R be a ring. We say that R is graded, if there is a direct sum decomposition, M R = Rn; n2N where each Rn is an additive subgroup of R, such that RdRe ⊂ Rd+e: The elements of Rd are called the homogeneous elements of order d. Let R be a graded ring. We say that an R-module M is graded if there is a direct sum decomposition M M = Mn; n2N compatible with the grading on R in the obvious way, RdMn ⊂ Md+n: A morphism of graded modules is an R-module map φ: M −! N of graded modules, which respects the grading, φ(Mn) ⊂ Nn: A graded submodule is a submodule for which the inclusion map is a graded morphism. A graded ideal I of R is an ideal, which when considered as a submodule is a graded submodule. Note that the kernel and cokernel of a morphism of graded modules is a graded module. Note also that an ideal is a graded ideal iff it is generated by homogeneous elements. Here is the key example. Example 3.2. Let R be the polynomial ring over a ring S. Define a direct sum decomposition of R by taking Rn to be the set of homogeneous polynomials of degree n. Given a graded ideal I in R, that is an ideal generated by homogeneous elements of R, the quotient is a graded ring. We will also need the notion of localisation, which is a straightfor- ward generalisation of the notion of the field of fractions. -
Associated Graded Rings Derived from Integrally Closed Ideals And
PUBLICATIONS MATHÉMATIQUES ET INFORMATIQUES DE RENNES MELVIN HOCHSTER Associated Graded Rings Derived from Integrally Closed Ideals and the Local Homological Conjectures Publications des séminaires de mathématiques et informatique de Rennes, 1980, fasci- cule S3 « Colloque d’algèbre », , p. 1-27 <http://www.numdam.org/item?id=PSMIR_1980___S3_1_0> © Département de mathématiques et informatique, université de Rennes, 1980, tous droits réservés. L’accès aux archives de la série « Publications mathématiques et informa- tiques de Rennes » implique l’accord avec les conditions générales d’utili- sation (http://www.numdam.org/conditions). Toute utilisation commerciale ou impression systématique est constitutive d’une infraction pénale. Toute copie ou impression de ce fichier doit contenir la présente mention de copyright. Article numérisé dans le cadre du programme Numérisation de documents anciens mathématiques http://www.numdam.org/ ASSOCIATED GRADED RINGS DERIVED FROM INTEGRALLY CLOSED IDEALS AND THE LOCAL HOMOLOGICAL CONJECTURES 1 2 by Melvin Hochster 1. Introduction The second and third sections of this paper can be read independently. The second section explores the properties of certain "associated graded rings", graded by the nonnegative rational numbers, and constructed using filtrations of integrally closed ideals. The properties of these rings are then exploited to show that if x^x^.-^x^ is a system of paramters of a local ring R of dimension d, d •> 3 and this system satisfies a certain mild condition (to wit, that R can be mapped -
Varieties of Quasigroups Determined by Short Strictly Balanced Identities
Czechoslovak Mathematical Journal Jaroslav Ježek; Tomáš Kepka Varieties of quasigroups determined by short strictly balanced identities Czechoslovak Mathematical Journal, Vol. 29 (1979), No. 1, 84–96 Persistent URL: http://dml.cz/dmlcz/101580 Terms of use: © Institute of Mathematics AS CR, 1979 Institute of Mathematics of the Czech Academy of Sciences provides access to digitized documents strictly for personal use. Each copy of any part of this document must contain these Terms of use. This document has been digitized, optimized for electronic delivery and stamped with digital signature within the project DML-CZ: The Czech Digital Mathematics Library http://dml.cz Czechoslovak Mathematical Journal, 29 (104) 1979, Praha VARIETIES OF QUASIGROUPS DETERMINED BY SHORT STRICTLY BALANCED IDENTITIES JAROSLAV JEZEK and TOMAS KEPKA, Praha (Received March 11, 1977) In this paper we find all varieties of quasigroups determined by a set of strictly balanced identities of length ^ 6 and study their properties. There are eleven such varieties: the variety of all quasigroups, the variety of commutative quasigroups, the variety of groups, the variety of abelian groups and, moreover, seven varieties which have not been studied in much detail until now. In Section 1 we describe these varieties. A survey of some significant properties of arbitrary varieties is given in Section 2; in Sections 3, 4 and 5 we assign these properties to the eleven varieties mentioned above and in Section 6 we give a table summarizing the results. 1. STRICTLY BALANCED QUASIGROUP IDENTITIES OF LENGTH £ 6 Quasigroups are considered as universal algebras with three binary operations •, /, \ (the class of all quasigroups is thus a variety). -
Commutative Algebra, Lecture Notes
COMMUTATIVE ALGEBRA, LECTURE NOTES P. SOSNA Contents 1. Very brief introduction 2 2. Rings and Ideals 2 3. Modules 10 3.1. Tensor product of modules 15 3.2. Flatness 18 3.3. Algebras 21 4. Localisation 22 4.1. Local properties 25 5. Chain conditions, Noetherian and Artin rings 29 5.1. Noetherian rings and modules 31 5.2. Artin rings 36 6. Primary decomposition 38 6.1. Primary decompositions in Noetherian rings 42 6.2. Application to Artin rings 43 6.3. Some geometry 44 6.4. Associated primes 45 7. Ring extensions 49 7.1. More geometry 56 8. Dimension theory 57 8.1. Regular rings 66 9. Homological methods 68 9.1. Recollections 68 9.2. Global dimension 71 9.3. Regular sequences, global dimension and regular rings 76 10. Differentials 83 10.1. Construction and some properties 83 10.2. Connection to regularity 88 11. Appendix: Exercises 91 References 100 1 2 P. SOSNA 1. Very brief introduction These are notes for a lecture (14 weeks, 2×90 minutes per week) held at the University of Hamburg in the winter semester 2014/2015. The goal is to introduce and study some basic concepts from commutative algebra which are indispensable in, for instance, algebraic geometry. There are many references for the subject, some of them are in the bibliography. In Sections 2-8 I mostly closely follow [2], sometimes rearranging the order in which the results are presented, sometimes omitting results and sometimes giving statements which are missing in [2]. In Section 9 I mostly rely on [9], while most of the material in Section 10 closely follows [4]. -
MAXIMAL and NON-MAXIMAL ORDERS 1. Introduction Let K Be A
MAXIMAL AND NON-MAXIMAL ORDERS LUKE WOLCOTT Abstract. In this paper we compare and contrast various prop- erties of maximal and non-maximal orders in the ring of integers of a number field. 1. Introduction Let K be a number field, and let OK be its ring of integers. An order in K is a subring R ⊆ OK such that OK /R is finite as a quotient of abelian groups. From this definition, it’s clear that there is a unique maximal order, namely OK . There are many properties that all orders in K share, but there are also differences. The main difference between a non-maximal order and the maximal order is that OK is integrally closed, but every non-maximal order is not. The result is that many of the nice properties of Dedekind domains do not hold in arbitrary orders. First we describe properties common to both maximal and non-maximal orders. In doing so, some useful results and constructions given in class for OK are generalized to arbitrary orders. Then we de- scribe a few important differences between maximal and non-maximal orders, and give proofs or counterexamples. Several proofs covered in lecture or the text will not be reproduced here. Throughout, all rings are commutative with unity. 2. Common Properties Proposition 2.1. The ring of integers OK of a number field K is a Noetherian ring, a finitely generated Z-algebra, and a free abelian group of rank n = [K : Q]. Proof: In class we showed that OK is a free abelian group of rank n = [K : Q], and is a ring. -
October 2013
LONDONLONDON MATHEMATICALMATHEMATICAL SOCIETYSOCIETY NEWSLETTER No. 429 October 2013 Society MeetingsSociety 2013 ELECTIONS voting the deadline for receipt of Meetings TO COUNCIL AND votes is 7 November 2013. and Events Members may like to note that and Events NOMINATING the LMS Election blog, moderated 2013 by the Scrutineers, can be found at: COMMITTEE http://discussions.lms.ac.uk/ Thursday 31 October The LMS 2013 elections will open on elections2013/. Good Practice Scheme 10th October 2013. LMS members Workshop, London will be contacted directly by the Future elections page 15 Electoral Reform Society (ERS), who Members are invited to make sug- Friday 15 November will send out the election material. gestions for nominees for future LMS Graduate Student In advance of this an email will be elections to Council. These should Meeting, London sent by the Society to all members be addressed to Dr Penny Davies 1 page 4 who are registered for electronic who is the Chair of the Nominat- communication informing them ing Committee (nominations@lms. Friday 15 November that they can expect to shortly re- ac.uk). Members may also make LMS AGM, London ceive some election correspondence direct nominations: details will be page 5 from the ERS. published in the April 2014 News- Monday 16 December Those not registered to receive letter or are available from Duncan SW & South Wales email correspondence will receive Turton at the LMS (duncan.turton@ Regional Meeting, all communications in paper for- lms.ac.uk). Swansea mat, both from the Society and 18-21 December from the ERS. Members should ANNUAL GENERAL LMS Prospects in check their post/email regularly in MEETING Mathematics, Durham October for communications re- page 11 garding the elections. -
Unique Factorization of Ideals in a Dedekind Domain
UNIQUE FACTORIZATION OF IDEALS IN A DEDEKIND DOMAIN XINYU LIU Abstract. In abstract algebra, a Dedekind domain is a Noetherian, inte- grally closed integral domain of Krull dimension 1. Parallel to the unique factorization of integers in Z, the ideals in a Dedekind domain can also be written uniquely as the product of prime ideals. Starting from the definitions of groups and rings, we introduce some basic theory in commutative algebra and present a proof for this theorem via Discrete Valuation Ring. First, we prove some intermediate results about the localization of a ring at its maximal ideals. Next, by the fact that the localization of a Dedekind domain at its maximal ideal is a Discrete Valuation Ring, we provide a simple proof for our main theorem. Contents 1. Basic Definitions of Rings 1 2. Localization 4 3. Integral Extension, Discrete Valuation Ring and Dedekind Domain 5 4. Unique Factorization of Ideals in a Dedekind Domain 7 Acknowledgements 12 References 12 1. Basic Definitions of Rings We start with some basic definitions of a ring, which is one of the most important structures in algebra. Definition 1.1. A ring R is a set along with two operations + and · (namely addition and multiplication) such that the following three properties are satisfied: (1) (R; +) is an abelian group. (2) (R; ·) is a monoid. (3) The distributive law: for all a; b; c 2 R, we have (a + b) · c = a · c + b · c, and a · (b + c) = a · b + a · c: Specifically, we use 0 to denote the identity element of the abelian group (R; +) and 1 to denote the identity element of the monoid (R; ·). -
CRITERIA for FLATNESS and INJECTIVITY 3 Ring of R
CRITERIA FOR FLATNESS AND INJECTIVITY NEIL EPSTEIN AND YONGWEI YAO Abstract. Let R be a commutative Noetherian ring. We give criteria for flatness of R-modules in terms of associated primes and torsion-freeness of certain tensor products. This allows us to develop a criterion for regularity if R has characteristic p, or more generally if it has a locally contracting en- domorphism. Dualizing, we give criteria for injectivity of R-modules in terms of coassociated primes and (h-)divisibility of certain Hom-modules. Along the way, we develop tools to achieve such a dual result. These include a careful analysis of the notions of divisibility and h-divisibility (including a localization result), a theorem on coassociated primes across a Hom-module base change, and a local criterion for injectivity. 1. Introduction The most important classes of modules over a commutative Noetherian ring R, from a homological point of view, are the projective, flat, and injective modules. It is relatively easy to check whether a module is projective, via the well-known criterion that a module is projective if and only if it is locally free. However, flatness and injectivity are much harder to determine. R It is well-known that an R-module M is flat if and only if Tor1 (R/P,M)=0 for all prime ideals P . For special classes of modules, there are some criteria for flatness which are easier to check. For example, a finitely generated module is flat if and only if it is projective. More generally, there is the following Local Flatness Criterion, stated here in slightly simplified form (see [Mat86, Section 22] for a self-contained proof): Theorem 1.1 ([Gro61, 10.2.2]). -
Splitting of Vector Bundles on Punctured Spectrum of Regular Local Rings
City University of New York (CUNY) CUNY Academic Works All Dissertations, Theses, and Capstone Projects Dissertations, Theses, and Capstone Projects 2005 Splitting of Vector Bundles on Punctured Spectrum of Regular Local Rings Mahdi Majidi-Zolbanin Graduate Center, City University of New York How does access to this work benefit ou?y Let us know! More information about this work at: https://academicworks.cuny.edu/gc_etds/1765 Discover additional works at: https://academicworks.cuny.edu This work is made publicly available by the City University of New York (CUNY). Contact: [email protected] Splitting of Vector Bundles on Punctured Spectrum of Regular Local Rings by Mahdi Majidi-Zolbanin A dissertation submitted to the Graduate Faculty in Mathematics in partial fulfillment of the requirements for the degree of Doctor of Philosophy, The City University of NewYork. 2005 UMI Number: 3187456 Copyright 2005 by Majidi-Zolbanin, Mahdi All rights reserved. UMI Microform 3187456 Copyright 2005 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, MI 48106-1346 ii c 2005 Mahdi Majidi-Zolbanin All Rights Reserved iii This manuscript has been read and accepted for the Graduate Faculty in Mathematics in satisfaction of the dissertation requirements for the degree of Doctor of Philosophy. Lucien Szpiro Date Chair of Examining Committee Jozek Dodziuk Date Executive Officer Lucien Szpiro Raymond Hoobler Alphonse Vasquez Ian Morrison Supervisory Committee THE CITY UNIVERSITY OF NEW YORK iv Abstract Splitting of Vector Bundles on Punctured Spectrum of Regular Local Rings by Mahdi Majidi-Zolbanin Advisor: Professor Lucien Szpiro In this dissertation we study splitting of vector bundles of small rank on punctured spectrum of regular local rings. -
Depth, Dimension and Resolutions in Commutative Algebra
Depth, Dimension and Resolutions in Commutative Algebra Claire Tête PhD student in Poitiers MAP, May 2014 Claire Tête Commutative Algebra This morning: the Koszul complex, regular sequence, depth Tomorrow: the Buchsbaum & Eisenbud criterion and the equality of Aulsander & Buchsbaum through examples. Wednesday: some elementary results about the homology of a bicomplex Claire Tête Commutative Algebra I will begin with a little example. Let us consider the ideal a = hX1, X2, X3i of A = k[X1, X2, X3]. What is "the" resolution of A/a as A-module? (the question is deliberatly not very precise) Claire Tête Commutative Algebra I will begin with a little example. Let us consider the ideal a = hX1, X2, X3i of A = k[X1, X2, X3]. What is "the" resolution of A/a as A-module? (the question is deliberatly not very precise) We would like to find something like this dm dm−1 d1 · · · Fm Fm−1 · · · F1 F0 A/a with A-modules Fi as simple as possible and s.t. Im di = Ker di−1. Claire Tête Commutative Algebra I will begin with a little example. Let us consider the ideal a = hX1, X2, X3i of A = k[X1, X2, X3]. What is "the" resolution of A/a as A-module? (the question is deliberatly not very precise) We would like to find something like this dm dm−1 d1 · · · Fm Fm−1 · · · F1 F0 A/a with A-modules Fi as simple as possible and s.t. Im di = Ker di−1. We say that F· is a resolution of the A-module A/a Claire Tête Commutative Algebra I will begin with a little example. -
Gsm073-Endmatter.Pdf
http://dx.doi.org/10.1090/gsm/073 Graduat e Algebra : Commutativ e Vie w This page intentionally left blank Graduat e Algebra : Commutativ e View Louis Halle Rowen Graduate Studies in Mathematics Volum e 73 KHSS^ K l|y|^| America n Mathematica l Societ y iSyiiU ^ Providence , Rhod e Islan d Contents Introduction xi List of symbols xv Chapter 0. Introduction and Prerequisites 1 Groups 2 Rings 6 Polynomials 9 Structure theories 12 Vector spaces and linear algebra 13 Bilinear forms and inner products 15 Appendix 0A: Quadratic Forms 18 Appendix OB: Ordered Monoids 23 Exercises - Chapter 0 25 Appendix 0A 28 Appendix OB 31 Part I. Modules Chapter 1. Introduction to Modules and their Structure Theory 35 Maps of modules 38 The lattice of submodules of a module 42 Appendix 1A: Categories 44 VI Contents Chapter 2. Finitely Generated Modules 51 Cyclic modules 51 Generating sets 52 Direct sums of two modules 53 The direct sum of any set of modules 54 Bases and free modules 56 Matrices over commutative rings 58 Torsion 61 The structure of finitely generated modules over a PID 62 The theory of a single linear transformation 71 Application to Abelian groups 77 Appendix 2A: Arithmetic Lattices 77 Chapter 3. Simple Modules and Composition Series 81 Simple modules 81 Composition series 82 A group-theoretic version of composition series 87 Exercises — Part I 89 Chapter 1 89 Appendix 1A 90 Chapter 2 94 Chapter 3 96 Part II. AfRne Algebras and Noetherian Rings Introduction to Part II 99 Chapter 4. Galois Theory of Fields 101 Field extensions 102 Adjoining