无单位元的群分次环、Smash积及其一个应用

本文对于具有局部单位元的群分次环Ｒ证明了左Ｒ＃Ｃ^*－模范畴与分次左Ｒ－模范畴是同构的，并给出Ｒ是分次右完全环的一些充要条件．     

Finite Gorenstein representation type implies simple singularity

Let R be a commutative noetherian local ring and consider the set of isomorphism classes of indecomposable totally reflexive R-modules. We prove that if this set is finite, then either it has exactly one element, represented by the rank 1 free module, or R is Gorenstein and an isolated singularity (if R is complete, then it is even a simple hypersurface singularity). The crux of our proof is to argue that if the residue field has a totally reflexive cover, then R is Gorenstein or every totally reflexive R-module is free.     

A characterization of the determinant

In this paper we examine group morphisms Λ: GL_n(R) → R^* from the general linear group over a commutative ring R into the group of units R^* of R and ask, "When are these morphisms functions of the determinant?"     

Automorphisms of the standard Borel subalgebra of Lie algebra of Cm type over a commutative ring

All automorphisms of the standard Borel subalgebra of the symplectic algebra sp(2m, R) are determined, provided that R is a commutative ring with identity, 2 is invertible in R.     

On the sum powers of matrices

A theorem of Lagrange says that every natural number is the sum of 4 squares. M. Newman proved that every integral n by n matrix is the sum of 8 (-1)ⁿ squares when n is at least 2. He asked to generalize this to the rings of integers of algebraic number fields. We show that an n by n matrix over a a commutative R with 1 is the sum of squares if and only if its trace reduced modulo 2Ris a square in the ring R/2R. It this is the case (and n is at least 2), then the matrix is the sum of 6 squares (5 squares would do when n is even). Moreover, we obtain a similar result for an arbitrary ring R with 1. Answering another question of M. Newman, we show that every integral n by n matrix is the sum of ten k-th powers for all sufficiently large n. (depending on k).     

Zero-dimensionality and the GE2 of polynomial rings

A technique of Pierce is used to prove that any commutative ring R, the polynomial ring R[X] is a GE₂-ring if and only if R is zero-dimensional. The technique yields other deductions about zero-dimensional rings as well.     

Brauer–Thrall for Totally Reflexive Modules over Local Rings of Higher Dimension

Let R be a commutative Noetherian local ring. Assume that R has a pair {x,y} of exact zerodivisors such that dim R/(x,y)?≥?2 and all totally reflexive R/(x)-modules are free. We show that the first and second Brauer–Thrall type theorems hold for the category of totally reflexive R-modules. More precisely, we prove that, for infinitely many integers n, there exists an indecomposable totally reflexive R-module of multiplicity n. Moreover, if the residue field of R is infinite, we prove that there exist infinitely many isomorphism classes of indecomposable totally reflexive R-modules of multiplicity n.     

The Extensional Structure of Commutative Noetherian Rings

Abstract

We show that finitely generated modules over a commutative Noetherian ring can be classified, up to isomorphism of submodule series, in a manner analogous to the classification of integers as products of prime numbers. In outline, two such modules have isomorphic submodule series if and only if 1) the set of minimal associated prime ideals of these modules coincide, 2) the multiplicities of these modules at these prime ideals coincide, and 3) the modules represent the same element in a certain group corresponding to the above set of prime ideals. Regarding condition 3), we show that, in the very special case that the ring is a Dedekind domain, the group corresponding to the prime ideal (0) is the ideal class group of the ring.     

On a problem about face polynomials

It is proved that an R-automorphism of polynomial ring R[x₁,…,x_n] is completely determined by its face polynomials, where R is a reduced commutative ring and n≥2. An example is given which shows that the condition R being reduced cannot be weakened.     

Generating sets for rings of algebraic integers

Let R be the ring of integers of some finite algebraic extension of the rationals Q of degree n. A necessary and sufficient condition for s elements of R to be an R-basis is given, in terms of the Hermite normal form of a certain n ×ns integral matrix depending on the elements, and on the structure constants of R.     

Isomorphism classes and adjoints of certain iwasawa modules

Letp be an odd prime number andO the integer ring of a finite extension of ℚ _p . We determine isomorphism classes of certainO[[T]]-modules which are isomorphic toO ^⊕3 asO-modules. Moreover we give some examples which are not isomorphic to their adjoints. Partly supported by the Grants-in-Aid for Encouragement of Young Scientists (No. 11740020), The Ministry of Education, Science, Sports and Culture of Japan.     

Classifying subcategories of finitely generated modules over a Noetherian ring

For R a commutative Noetherian ring, wide and Serre subcategories of finitely generated R-modules have been classified by their support. This paper studies general torsion classes and introduces narrow subcategories. These are closed under fewer operations than wide and Serre subcategories, but still for finitely generated R-modules both narrow subcategories and torsion classes are classified using the same support data. Although for finitely generated R-modules all four kinds of subcategories coincide, they do not coincide in the larger category of all R-modules.     

On the Image of the Totaling Functor

Let A be a differential graded (DG) algebra with a trivial differential over a commutative unital ring. This paper investigates the image of the totaling functor, defined from the category of complexes of graded A-modules to the category of DG A-modules. Specifically, we exhibit a special class of semifree DG A-modules which can always be expressed as the totaling of some complex of graded free A-modules. As a corollary, we also provide results concerning the image of the totaling functor when A is a polynomial ring over a field.     

K0 of Purely Infinite Simple Regular Rings

We extend the notion of a purely infinite simple C ^*-algebra to the context of unital rings, and we study its basic properties, specially those related to K-theory. For instance, if R is a purely infinite simple ring, then K ₀(R)⁺ = K ₀(R), the monoid of isomorphism classes of finitely generated projective R-modules is isomorphic to the monoid obtained from K ₀(R) by adjoining a new zero element, and K ₁(R) is the Abelianization of the group of units of R. We develop techniques of construction, obtaining new examples in this class in the case of von Neumann regular rings, and we compute the Grothendieck groups of these examples. In particular, we prove that every countable Abelian group is isomorphic to K ₀ of some purely infinite simple regular ring. Finally, some known examples are analyzed within this framework.     

A stable range for quadratic forms over commutative rings

A commutative ring A has quadratic stable range 1 (qsr(A) = 1) if each primitive binary quadratic form over A represents a unit. It is shown that qsr(A) = 1 implies that every primitive quadratic form over A represents a unit, A has stable range 1 and finitely generated constant rank projectives over A are free. A classification of quadratic forms is provided over Bezout domains with characteristic other than 2, quadratic stable range 1, and a strong approximation property for a certain subset of their maximum spectrum. These domains include rings of holomorphic functions on connected noncompact Riemann surfaces. Examples of localizations of rings of algebraic integers are provided to show that the classical concept of stable range does not behave well in either direction under finite integral extensions and that qsr(A) = 1 does not descend from such extensions.     

Algebraic properties of isometries between groups of invertible elements in Banach algebras

We prove that an isometry T between open subgroups of the invertible groups of unital Banach algebras A and B is extended to a real-linear isometry up to translation between these Banach algebras. While a unital isometry between unital semisimple commutative Banach algebras need not be multiplicative, we prove in this paper that if A is commutative and A or B are semisimple, then (T(eA))⁻¹T is extended to an isometric real algebra isomorphism from A onto B. In particular, A−1 is isometric as a metric space to B−1 if and only if they are isometrically isomorphic to each other as metrizable groups if and only if A is isometrically isomorphic to B as a real Banach algebra; it is compared by the example of ?elazko concerning on non-isomorphic Banach algebras with the homeomorphically isomorphic invertible groups. Isometries between open subgroups of the invertible groups of unital closed standard operator algebras on Banach spaces are investigated and their general forms are given.     

Ring structures on groups of arithmetic functions

Using the theory of Witt vectors, we define ring structures on several well-known groups of arithmetic functions, which in another guise are formal Dirichlet series. The set of multiplicative arithmetic functions over a commutative ring R is shown to have a unique functorial ring structure for which the operation of addition is Dirichlet convolution and the operation of multiplication restricted to the completely multiplicative functions coincides with point-wise multiplication. The group of additive arithmetic functions over R also has a functorial ring structure. In analogy with the ghost homomorphism of Witt vectors, there is a functorial ring homomorphism from the ring of multiplicative functions to the ring of additive functions that is an isomorphism if R is a Q-algebra. The group of rational arithmetic functions, that is, the group generated by the completely multiplicative functions, forms a subring of the ring of multiplicative functions. The latter ring has the structure of a Bin(R)-algebra, where Bin(R) is the universal binomial ring equipped with a ring homomorphism to R. We use this algebra structure to study the order of a rational arithmetic function, as well the powersfα for α∈Bin(R) of a multiplicative arithmetic function f. For example, we prove new results about the powers of a given multiplicative arithmetic function that are rational. Finally, we apply our theory to the study of the zeta function of a scheme of finite type over Z.     

Polarity and transitive parabolic unitals in translation planes of odd order

A parabolic unital of a translation plane is called transitive, if the collineation group G fixing fixes the point at infinity of and acts transitively on the affine points of . It has been conjectured that if a transitive parabolic unital consists of the absolute points of a unitary polarity in a commutative semi-field plane, then the sharply transitive normal subgroupK of G is not commutative. So far, this has been proved for commutative twisted field planes of odd square order, see [1],[5]. Here we prove this conjecture for commutative Dickson planes. Received 14 May 2001.     

Structure of rings of functions with Riemann Stieltjes convolution products

In this note three sets of complex valued functions with pointwise addition and a Riemann Stieltjes convolution product are considered. The functions considered are discrete analytic functions, sequences, and continuous functions of bounded variation defined on the nonnegative real numbers. Each forms a commutative algebra with identity. The discrete analytic functions form a principal ideal ring with five maximal ideals, nine prime ideals, and is essentially a direct sum of four discrete valuation rings. The ring of sequences is isomorphic to an ideal of the ring of discrete analytic functions; it has two maximal and three prime ideals. Both contain divisors of zero. The units, associates, irreducible elements and primes in these two rings are described. The results are used to study the continuous functions; partial results are obtained concerning units and divisors of zero. The product satisfies a convolution theorem.