Direct-Sum Decompositions over Local Rings

The direct sum behaviour of its projective modules is a fundamental property of any ring. Hereditary Noetherian prime rings are perhaps the only noncommutative Noetherian rings for which this direct sum behaviour (for both finitely and infinitely generated projective modules) is well-understood, yet highly nontrivial . This book surveys material previously available only in the research literature. It provides a re-worked and simplified account, with improved clarity, fresh insights and many original results about finite length modules, injective modules and projective modules. It culminates in the authors' surprisingly complete structure theorem for projective modules which involves two independent additive invariants: genus and Steinitz class. Several applications demonstrate its utility. The theory, extending the well-known module theory of commutative Dedekind domains and of hereditary orders, develops via a detailed study of simple modules. This relies upon the substantial account of idealiser subrings which forms the first part of the book and provides a useful general construction tool for interesting examples. The book assumes some knowledge of noncommutative Noetherian rings, including Goldie's theorem. Beyond that, it is largely self-contained, thanks to the appendix which provides succinct accounts of Artinian serial rings and, for arbitrary rings, results about lifting direct sum decompositions from finite length images of projective modules. The appendix also describes some open problems.

We prove a theorem on the intersection theory over a Noetherian local ring , which gives a new proof of a classical theorem of Rees about degree functions. To obtain this, we define an intersection product on schemes that are proper and birational over such rings , using the theory of rational equivalence developed by Thorup, and the Snapper–Mumford–Kleiman intersection theory for proper schemes over an Artinian local ring. Our development of this product is essentially self‐contained. As a central component of the proof of our main theorem, we extend to arbitrary Noetherian local rings a formula by Ramanujam that computes Hilbert–Samuel multiplicities. In the final section, we express mixed multiplicities in terms of intersection theory and conclude from this that they satisfy a certain multilinearity condition. Then we interpret some theorems of Rees and Sharp and of Teissier about mixed multiplicities over 2‐dimensional excellent local rings in terms of our intersection product.

Local rings whose modules are almost serial

Let (X, x O) be a normal complex analytic space andA⋐X a connected Stein compact set, i.e. a compact subset ofX which has a basis of open neighborhoods which are Stein spaces. We restrict attention to thoseA such thatR=H 0 O) is Noetherian. In Section I various exact sequences involving the divisor class group ofR, denotedC(R), are developed. (IfA is a point, one of these sequences is well-known [24], [39].) LetB be a connected compact Stein set on a normal varietyY such thatS=H o(B, Y O) andT=H o(A×B, X×Y O are Noetherian. In II we give a Kunneth-type formula which relatesC(R), C(S) andC(T). In III we show certain analytic local rings are unique factorization domains, study the divisor class groups of local rings on the quotient of an analytic space by a finite group, and prove a simple result on the topology of germs of complex analytic sets. We give a function-theoretic proof that complete intersections of dimensions greater than three which have isolated singularities have local rings which are unique factorization domains.

This paper presents a comprehensive study of right $${\textbf {S}}$$ S -maximal ideals in noncommutative rings, providing a natural extension of classical ideal theory through the framework of $${\textbf {m}}$$ m -systems. We establish deep connections among right $${\textbf {S}}$$ S -maximal, S -comaximal, and $${\textbf {S}}$$ S -prime ideals, and introduce new concepts such as the $${\textbf {S}}$$ S -Jacobson radical and $${\textbf {S}}$$ S -invertible elements to further develop the structural theory of rings. A key contribution is the introduction of $${\textbf {S}}$$ S -local rings and the formulation of an $${\textbf {S}}$$ S -version of Nakayama’s Lemma. In addition, we propose two complementary definitions of left $${\textbf {S}}$$ S -primitive ideals–one ideal-theoretic and the other annihilator-based–and demonstrate their intrinsic relationship to $${\textbf {S}}$$ S -prime ideals. Our results not only unify and generalize existing notions but also open promising avenues for further research, particularly in exploring the interplay between different forms of $${\textbf {S}}$$ S -primitivity.

The content in Chapter 1–3 is a fairly standard one-semester course on local rings with the goal to reach the fact that a regular local ring is a unique factorization domain. The homological machinery is also supported by Cohen–Macaulay rings and depth. In Chapters 4–6 the methods of injective modules, Matlis duality and local cohomology are discussed. Chapters 7–9 are not so standard and introduce the reader to the generalizations of modules to complexes of modules. Some of Professor Iversen's results are given in Chapter 9. Chapter 10 is about Serre's intersection conjecture. The graded case is fully exposed. The last chapter introduces the reader to Fitting ideals and McRae invariants.

This series aims to report new developments in mathematical research and teaching -quickly, informally and at a high level.The type of material considered for publication includes: 1.Preliminary drafts of original papers and monographs 2. Lectures on a new field, or presenting a new angle on a classical field 3. Seminar work-outs 4. Reports of meetings, provided they are a) of exceptional interest or b) devoted to a single topic.

Let A be a (commutative, Noetherian) local ring (with identity) and let a1,…, an be a system of parameters (s.o.p.) for A. A (not necessarily finitely generated) A-module M is said to be a big Cohen–Macaulay.A-module with respect to a1,…, an if a1,…, an is an M-sequence, that is if M ‡ = (a1,…, an) M and, for each i = 1,…, n,One of the main open problems in commutative algebra at the present time is that of establishing the existence of a big Cohen–Macaulay module with respect to a specified s.o.p. in an arbitrary local ring. The work and writings of Hochster, such as (5), show that, if the existence of such modules could be established, then several conjectures in commutative algebra, some of which are quite long-standing, would be settled. Moreover, Hochster has established the existence of such big Cohen–Macaulay modules whenever the local ring A concerned contains a field as a subring, or has dimension not exceeding 2: see ((5), chapters 4, 5) and (4).

Completions of Local Rings with an Isolated Singularity

Blowing up finitely supported complete ideals in a regular local ring

Keskin and Tribak (2005) studied the structure of weak lifting modules with small radicals over commutative noetherian (local) rings. They proved that a module M is weak lifting if and only if M is a direct sum of local modules of some special type. Such modules were further studied by Tribak (2007). In this note we study weak lifting modules M with small radical over arbitrary rings. We prove that M is an irredundant sum M=Σ i∈I M i where each M i is local and Σ i∈F M i is a summand of M for every finite subset F of I. Moreover Σ i∈F M i = ⊕ i∈F K i with K i local. In particular a finitely generated weak lifting module is a direct sum of local modules. This generalizes the analogous result for finitely generated lifting modules.

This paper studies the computational complexity of subclasses of semiperfect rings from the perspective of computability theory. A ring is semiperfect if the identity can be expressed as a sum of mutually orthogonal local idempotents. Semisimple rings and local rings are typical subclasses of semiperfect rings that play important roles in noncommutative algebra. First, we define a ring to be semisimple if the left regular module can be decomposed as a finite direct sum of simple submodules and prove that the index set of computable semisimple rings is Σ20-hard within the index set of computable rings. Second, we define local rings by using equivalent properties of non-left invertible elements of rings and show that the index set of computable local rings is Π20-hard within the index set of computable rings. Finally, based on the Π20 definition of local rings, computable semiperfect rings can be described by Σ30 formulas. As a corollary, we find that the index set of computable semiperfect rings can be both Σ20-hard and Π20-hard within the index set of computable rings.

On modular cyclic codes

In this paper, we study the decompositions of an amply supplemented module M into a direct sum of cotype submodules. For this purpose, we introduce some conatural classes, such as the class of all q-molecular modules, the class of all q-bottomless modules, the class of all q-discrete modules and the class of all q-continuous modules, which are defined for every ring. We also find conditions on the structure of the cotype submodules of an amply supplemented module M, in order for M to have certain direct sum decompositions.

<p>The goal of this thesis is to study non-commutative deformation rings of representations of algebras. The main motivation is to provide a generalization of the deformation theory over commutative local rings studied by B. Mazur, M. Schlessinger and others. The latter deformation theory has played an important role in number theory, and in particular in the proof of Fermat's Last Theorem.</p> <p>The thesis is divided into two parts.</p> <p>In the first part, <em>A</em> is an arbitrary λ-algebra for a complete local commutative Noetherian ring λ with residue field <em>k</em>. A category Ĉ is defined whose objects are complete local λ-algebras <em>R</em> with residue field <em>k</em> such that <em>R</em> is a quotient ring of a power series algebra over λ in finitely many non-commuting variables. If <em>V</em> is a finite dimensional <em>k</em>-vector space that is also a left <em>A</em>-module and that satisfies a natural finiteness condition, it is proved that <em>V</em> has a so-called versal deformation ring <em>R(A,V)</em>. More precisely, <em>R(A,V)</em> is an object in Ĉ such that the isomorphism class of every lift of <em>V</em> over an object <em>R</em> in Ĉ arises from a morphism α : <em>R(A,V)</em>→ <em>R</em> in Ĉ and α is unique if <em>R</em> is the ring of dual numbers <em>k</em>[ϵ].</p> <p>In the second part, two particular examples of λ, <em>A</em> and <em>V</em> are studied and the versal deformation ring <em>R(A,V)</em> is determined in each of these cases. In the first example, λ=<em>k</em>, <em>A</em> is a series of non-commutative <em>k</em>-algebras depending on a parameter <em>r≥2</em>, and <em>V</em> is a particular quotient module of <em>A</em>; it is shown that <em>R(A,V)</em> is isomorphic to <em>A</em>. The second example builds on the first example when <em>r=2</em> and uses that, if additionally the characteristic of <em>k</em> is 2, then <em>A</em> is isomorphic to the group ring <em>k[D<sub>8</sub>]</em> of a dihedral group <em>D<sub>8</sub></em> of order 8.</p> <p>It is shown that if <em>k</em> is perfect and <em>W</em> is the ring of infinite Witt vectors over <em>k</em>, then <em>R(W[D<sub>8</sub>],V)</em> is isomorphic to <em>W[D<sub>8</sub>]</em>.</p>