Free Pairs of Bicyclic Units in the Integral Group Ring of the Dihedral Group

Hirano studied the quasi-Armendariz property of rings, and then this concept was generalized by some authors, defining quasi-Armendariz property for skew polynomial rings and monoid rings. In this article, we consider unified approach to the quasi-Armendariz property of skew power series rings and skew polynomial rings by considering the quasi-Armendariz condition in mixed extension ring [R; I][x; σ], introducing the concept of so-called (σ, I)-quasi Armendariz ring, where R is an associative ring equipped with an endomorphism σ and I is an σ-stable ideal of R. We study the ring-theoretical properties of (σ, I)-quasi Armendariz rings, and we obtain various necessary or sufficient conditions for a ring to be (σ, I)-quasi Armendariz. Constructing various examples, we classify how the (σ, I)-quasi Armendariz property behaves under various ring extensions. Furthermore, we show that a number of interesting properties of an (σ, I)-quasi Armendariz ring R such as reflexive and quasi-Baer property transfer to its mixed extension ring and vice versa. In this way, we extend the well-known results about quasi-Armendariz property in ordinary polynomial rings and skew polynomial rings for this class of mixed extensions. We pay also a particular attention to quasi-Gaussian rings.     

Essential dimension of involutions and subalgebras

Essential dimension is an invariant of algebraic groups G over a field F that measures the complexity of G-torsors over field extensions of F. We use theorems of N. Karpenko about the incompressibility of Severi-Brauer varieties and quadratic Weil transfers of Severi-Brauer varieties to compute the essential dimension of some closed subgroups of R _K/F (GL ₁(A)), where A is a central division K-algebra of prime power degree and K/F is a separable field extension of degree ≤ 2. In particular, we determine the essential dimension of the group Sim(A, σ) of similitudes of (A, σ), where σ is an F-involution on A, and the essential dimension of the normalizer $N_{GL_1 (A)} \left( {GL_1 \left( B \right)} \right)$ , where B is a separable subalgebra of A.     

Agreeable domains

An integral domain D with quotient field K is defined to be agreeable if for each fractional ideal F of D[X] with F C K[X] there exists 0 = s ε D with sF C D[X]. D is agreeable ? D satisfies property (*) (for 0 ^ f(X) G K[X], there exists 0 = s ε D so that f(X)g(X) ε D[X] for g(X) ε K[X] implies that sg(X) ε D[X]) &; D[X] is an almost principal domain, i.e., for each nonzero ideal I of D[X] with IK[X] = K[X], there exists f(X) ε I and 0 = s ε D with sI C (f(X)). If D is Noetherian or integrally closed, then D is agreeable. A number of other characterizations of agreeable domains are given as are a number of stability properties. For example, if D is agreeable, so is ?^αD_{P α} and for a pair of domains D?D′ with a [DD:′]≠0, D is agreeable?D′ is agreeable. Results on agreeable domains are used to give an alternative treatment of Querre's characterization of divisorial ideals in integrally closed polynomial rings. Finally, the various characterizations of D being agreeable are considered for polynomial rings in several variables.     

A projective analogue of schur's tensor power construction

For a prime ring R with σ ? Aut(R), we examine the extended center of the skew power series ring R[[x;σ]]. We prove the extended center of R[[x;σ]] is isomorphic to: (1) C^σ if no power of σ is X-inner where C^σ is the fixed field of a acting on the extended center C of R (2) C^σ((vx')) (Laurent series) if some positive power of a is X-inner and R is closed prime. In this case I is the least positive integer such that a¹ is X-inner with σ¹= inn(v). We provide an example showing this result fails if R is not closed prime.     

Centralizers in Domains of Gelfand-Kirillov Dimension 2

Given an affine domain of Gelfand–Kirillov dimension 2over an algebraically closed field, it is shown that the centralizerof any non-scalar element of this domain is a commutative domainof Gelfand–Kirillov dimension 1 whenever the domain isnot polynomial identity. It is shown that the maximal subfieldsof the quotient division ring of a finitely graded Goldie algebraof Gelfand–Kirillov dimension 2 over a field F all havetranscendence degree 1 over F. Finally, centralizers of elementsin a finitely graded Goldie domain of Gelfand–Kirillovdimension 2 over an algebraically closed field are considered.In this case, it is shown that the centralizer of a non-scalarelement is an affine commutative domain of Gelfand–Kirillovdimension 1. 2000 Mathematics Subject Classification 16P90.     

Conditions on decomposable symmetric tensors as an algebraic variety

Let V be a finite dimensional vector space of dimension at least 2 over an infinite field F. We show that the set of all decomposable elements in the rth symmetric product space over i:V(r≥ 2) is an algebraic set if F is algebraically closed and only if every polynomial of degree at most r splits completcly over F.     

The centralizer of a semi-linear transformation

Let V be a finite-dimensional vector space over a division ring D, where D is finite-dimensional over its center F. Suppose T is a semi-linear transformation on V with associated automorphism σ of D. The centralizer of T is the ring C(T) of all linear transformations on V which commute with T. If σ^r is the identity on D for some r ? 1 and no smaller positive power of σ is an inner automorphism, then the center of C(T) is computed to be polynomials in T^r with coefficients from F₀, where F₀ is the subfield of F left elementwise fixed by σ. A matrix version of this theorem is also given.     

Free groups in a normal subgroup of the field of fractions of a skew polynomial ring

Let k(t) be the field of rational functions over the field k, let σ be a k-automorphism of K = k(t), let D = K(X;σ) be the ring of fractions of the skew polynomial ring K[X;σ], and let D^? be the multiplicative group of D. We show that if N is a noncentral normal subgroup of D^?, then N contains a free subgroup. We also prove that when k is algebraically closed and σ has infinite order, there exists a specialization from D to a quaternion algebra. This allows us to explicitly present free subgroups in D^?.     

The unique rank property among gabriel localizations

It is not uncommon for rings to have Gabriel localizations which do not possess the unique rank (UR) property although the rings themselves do have UR. We show that if F is a Gabriel filter of right ideals on a ring R and R_F is the corresponding Gabriel localization, then free R_F?modules of ranks m and n are isomorphic if and only if some F-dense submodule of (R/T_f(R))^m is isomorphic to some F-dense submodule of (R/T_F(R))ⁿ, where T_F(R) is the F-torsion ideal of R.     

A note on the multiplicity in factor ring

Let (R,m) = k[x ₁,..., x _n ](x ₁,...,x _n ) be a local polynomial ring (k being an algebraically closed field), and Q:= (F ₁,..., F _r )R be a primary ideal in R with respect to a maximal ideal m ⊂ R. In this short note we give a formula for the multiplicity e ₀ (QR/(F ₁)R, R/(F ₁)R). The author was supported by the grant No. 1/0262/03) of the Slovak Ministry of Education.     

On Additive Commutator Groups in Division Rings

Let D be a division ring with center F and denote by [D,D] the group generated additively by additive commutators. First, it is shown that in zero characteristic, D is algebraic over F if and only if each element of [D,D] is algebraic over F. We conjecture that this assertion is true for any characteristic. Also, as a generalization of Jacobson’s Theorem it is proved that D is an F-central division ring if and only if all its additive commutators are of bounded degree over F. Furthermore, we study the F- vector space D/[D,D] and show that dim_FD/[D,D] ≤ 1 if D is algebraic over F and char F = 0. We then prove that any algebraic division ring contains a separable additive commutator over F except in one special case. Finally, the existence of primitive elements in [D, D] is studied for finite separable extensions of F in D.     

Very ampleness of multiple of tetragonal linear systems

In this paper we continue the study of sandwich near‐rings. We introduce the sandwich near‐ring of homogeneous functions,N: = M _F(D,W,ψ) where Fis a fieldDis an F-setWa vector space over Fand ψW→D, a homogeneous map. We investigate the internal structure of Nin terms of the components D, W,and ψ     

Separable polynomials over finite dimensional algebras

In [5] it was shown that for a polynomial P of precise degree n with coefficients in an arbitrary m-ary algebra of dimension d as a vector space over an algebraically closed fields, the zeros of P together with the homogeneous zeros of the dominant part of P form a set of cardinality n^d or the cardinality of the base field. We investigate polynomials with coefficients in a d dimensional algebra A without assuming the base field k to be algebraically closed. Separable polynomials are defined to be those which have exactly n^d distinct zeros in [Ktilde] ?_k A [Ktilde] where [Ktilde] denotes an algebraic closure of k. The main result states that given a separable polynomial of degree n, the field extension L of minimal degree over k for which L ?_k A contains all nd zeros is finite Galois over k. It is shown that there is a non empty Zariski open subset in the affine space of all d-dimensional k algebras whose elements A have the following property: In the affine space of polynomials of precise degree n with coefficients in A there is a non empty Zariski open subset consisting of separable polynomials; in other polynomials with coefficients in a finite dimensional algebra are “generically” separable.     

On the Weyl Spectrum: Spectral Mapping Theorem and Weyl's Theorem

It is shown that ifTis a dominant operator or an analytic quasi-hyponormal operator on a complex Hilbert space and iffis a function analytic on a neighborhood of σ(T), then σ_w(f(T)) = f(σ_w(T)), where σ(T) and σ_w(T) stand respectively for the spectrum and the Weyl spectrum ofT; moreover, Weyl's theorem holds forf(T) + Fif “dominant” is replaced by “M-hyponormal,” whereFis any finite rank operator commuting withT. These generalize earlier results for hyponormal operators. It is also shown that there exist an operatorTand a finite rank operatorFcommuting withTsuch that Weyl's theorem holds forTbut not forT + F. This answers negatively a problem raised by K. K. Oberai (Illinois J. Math.21, 1977, 84–90). However, ifTis required to be isoloid, then the statement that Weyl's theorem holds forTwill imply it holds forT + F.     

Universal Deformation Rings of Modules over Self-injective Cluster-Tilted Algebras Are Trivial

Let k be a fixed algebraically closed field of arbitrary characteristic,let Λ be a finite dimensional self-injective k-algebra,and let V be an indecomposable non-projective left Λ-module with finite dimension over k.We prove that if τΛV is the Auslander-Reiten translation of V,then the versal deformation rings R(Λ,V)and R(Λ,τΛV)(in the sense of F.M.Bleher and the second author)are isomorphic.We use this to prove that if Λ is further a cluster-tilted k-algebra,then R(Λ,V)is universal and isomorphic to k.     

Links ore sets,and classical localizations

The notion of a simple ring D_Gderived from a group ring KG is introduced in case K is a field and G is an infinite residually finite group. The close link between D_Gand K_G is exploited in both directions: first, for a simple proof of the Kaplansky's conjecture concerning direct finiteness of KG. Second, to show that D_Gprovides counter-examples to some conjectures dealing with von Neumann regular rings and the rings all of whose one-sided ideals are generated by idempotents.     

Kronecker Function Rings of Transcendental Field Extensions

We consider the ring Kr(F/D), where D is a subring of a field F, that is the intersection of the trivial extensions to F(X) of the valuation rings of the Zariski–Riemann space consisting of all valuation rings of the extension F/D and investigate the ideal structure of Kr(F/D) in the case where D is an affine algebra over a subfield K of F and the extension F/K has countably infinite transcendence degree, by using the topological structure of the Zariski–Riemann space. We show that for any pair of nonnegative integers d and h, there are infinitely many prime ideals of dimension d and height h that are minimal over any proper nonzero finitely generated ideal of Kr(F/D).     

Mixed projection methods for systems of variational inequalities

Let H be a real Hilbert space. Let be bounded and continuous mappings where D(F) and D(K) are closed convex subsets of H. We introduce and consider the following system of variational inequalities: find [u ^*,v ^*]∈D(F) × D(K) such that This system of variational inequalities is closely related to a pseudomonotone variational inequality. The well-known projection method is extended to develop a mixed projection method for solving this system of variational inequalities. No invertibility assumption is imposed on F and K. The operators K and F also need not be defined on compact subsets of H.      

Injective homogeneity and homological homogeneity of the ore extensions

In this paper we prove that under some natural conditions, the Ore extensions and skew Laurent polynomial rings are injectively homogeneous or homologically homogeneous if so are their coefficient rings. Specifically, we prove that ifR is a commutative Noetherian ring of positive characteristic, thenA _n (R), then ^th Weyl algebra overR, is injectively homogeneous (resp. homologically homogeneous) ifR has finite injective dimension (resp. global dimension).