On Strongly Clean Matrix and Triangular Matrix Rings

A ring R with identity is called “clean” if every element of R is the sum of an idempotent and a unit, and R is called “strongly clean” if every element of R is the sum of an idempotent and a unit that commute. Strongly clean rings are “additive analogs” of strongly regular rings, where a ring R is strongly regular if every element of R is the product of an idempotent and a unit that commute. Strongly clean rings were introduced in Nicholson (1999 Nicholson , W. K. (1999). Strongly clean rings and Fitting's lemma. Comm. Algebra 27:3583–3592. [CSA] [Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]) where their connection with strongly π-regular rings and hence to Fitting's Lemma were discussed. Local rings and strongly π-regular rings are all strongly clean. In this article, we identify new families of strongly clean rings through matrix rings and triangular matrix rings. For instance, it is proven that the 2 × 2 matrix ring over the ring of p-adic integers and the triangular matrix ring over a commutative semiperfect ring are all strongly clean.     

A Question on Strongly Clean Rings

A new characterization of a strongly clean ring is given. And it is proven that if R is a strongly clean ring, then eRe is a strongly clean ring for e ² = e ∈ R, which answers a question of Nicholson (1999 Nicholson , W. K. ( 1999 ). Strongly clean rings and Fitting's lemma . Comm. Algebra 27 ( 8 ): 3583 – 3592 . [CSA] [Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]) in the affirmative.     

Quasipolar Property of Generalized Matrix Rings

The article concerns the question of when a generalized matrix ring K _s (R) over a local ring R is quasipolar. For a commutative local ring R, it is proved that K _s (R) is quasipolar if and only if it is strongly clean. For a general local ring R, some partial answers to the question are obtained. There exist noncommutative local rings R such that K _s (R) is strongly clean, but not quasipolar. Necessary and sufficient conditions for a single matrix of K _s (R) (where R is a commutative local ring) to be quasipolar is obtained. The known results on this subject in [5 Cui , J. , Chen , J. ( 2011 ). When is a 2 × 2 matrix ring over a commutative local ring quasipolar? Comm. Alg. 39 : 3212 – 3221 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]] are improved or extended.     

Strongly J-Clean Matrices Over Local Rings

An element of a ring is called strongly J-clean provided that it can be written as the sum of an idempotent and an element in its Jacobson radical that commute. We investigate, in this article, a single strongly J-clean 2 × 2 matrix over a noncommutative local ring. The criteria on strong J-cleanness of 2 × 2 matrices in terms of a quadratic equation are given. These extend the corresponding results in [8 Li , B. ( 2009 ). Strongly clean matrix rings over noncommutative local rings . Bull. Korean Math. Soc. 46 : 71 – 78 .[Crossref], [Web of Science ®] , [Google Scholar], Theorems 2.7 and 3.2], [9 Li , Y. ( 2007 ). Strongly clean matrix rings over local rings . J. Algebra 312 : 397 – 404 .[Crossref], [Web of Science ®] , [Google Scholar], Theorem 2.6], and [11 Yang , X. , Zhou , Y. ( 2008 ). Strongly cleanness of the 2 × 2 matrix ring over a general local ring . J. Algebra 320 : 2280 – 2290 .[Crossref], [Web of Science ®] , [Google Scholar], Theorem 7].     

Unimodular Regularity,Stable Range,and Exchange Rings

Let R be an exchange ring. In this article, we show that the following conditions are equivalent: (1) R has stable range not more than n; (2) whenever x ∈ R ⁿ is regular, there exists some unimodular regular w ∈  ⁿ R such that x = xwx; (3) whenever x ∈ R ⁿ is regular, there exist some idempotent e ∈ R and some unimodular regular w ∈ R ⁿ such that x = ew; (4) whenever x ∈ R ⁿ is regular, there exist some idempotent e ∈ M _n (R) and some unimodular regular w ∈ R ⁿ such that x = we; (5) whenever a( ⁿ R) + bR = dR with a ∈ R ⁿ and b,d ∈ R, there exist some z ∈ R ⁿ and some unimodular regular w ∈ R ⁿ such that a + bz = dw; (6) whenever x = xyx with x ∈ R ⁿ and y ∈  ⁿ R, there exist some u ∈ R ⁿ and v ∈  ⁿ R such that vxyu = yx and uv = 1. These, by replacing unimodularity with unimodular regularity, generalize the corresponding results of Canfell (1995 Canfell , M. J. ( 1995 ). Completions of diagrams by automorphisms and Bas' first stable range condition . J. Algebra 176 : 480 – 513 .[Crossref], [Web of Science ®] , [Google Scholar], Theorem 2.9), Chen (Chen 2000 Chen , H. ( 2000 ). On stable range conditions . Comm. Algebra 28 : 3913 – 3924 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar], Theorem 4.2 and Proposition 4.6, Chen 2001 Chen , H. ( 2001 ). Regular rings with finite stable range . Comm. Algebra 29 : 157 – 166 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar], Theorem 10), and Wu and Xu (1997 Wu , T. , Xu , Y. ( 1997 ). On the stable range condition of exchagne rings . Comm. Algebra 25 : 2355 – 2363 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar], Theorem 9), etc.     

Regularity in Endomorphism Rings

The unique largest regular ideal Reg(A, A) in the endomorphism ring End(A) is computed for abelian groups A using the general tools developed in [7 Kasch , F. , Mader , A. ( 2005 ). Regularity and substructures of hom . Communications in Algebra 34 : 1459 – 1478 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]]. This generalizes earlier results on groups with regular endomorphism ring. Interesting questions remain for a very special class of mixed abelian groups.     

Laurent Series Ring over Semiperfect Ring Can Not be Semiperfect

One of the main results of the article [2 Sonin , K. I. ( 1996 ). Semiprime and semiperfect rings of Laurent series . Mathematical Notes 60 : 222 – 226 .[Crossref], [Web of Science ®] , [Google Scholar]] says that, if a ring R is semiperfect and ? is an authomorphism of R, then the skew Laurent series ring R((x, ?)) is semiperfect. We will show that the above statement is not true. More precisely, we will show that, if the Laurent series ring R((x)) is semilocal, then R is semiperfect with nil Jacobson radical.     

On the almost Gorenstein property of determinantal rings

In this paper, we investigate the question of when the determinantal ring R over a field k is an almost Gorenstein local/graded ring in the sense of [14 Goto, S., Takahashi, R., Taniguchi, N. (2015). Almost Gorenstein rings - towards a theory of higher dimension. J. Pure Appl. Algebra 219:2666–2712.[Crossref], [Web of Science ®] , [Google Scholar]]. As a consequence of the main result, we see that if R is a non-Gorenstein almost Gorenstein local/graded ring, then the ring R has a minimal multiplicity.     

On Strongly Gorenstein FP-Injective Modules

This article continues to investigate a particular case of Gorenstein FP-injective modules, called strongly Gorenstein FP-injective modules. Some examples are given to show that strongly Gorenstein FP-injective modules lie strictly between FP-injective modules and Gorenstein FP-injective modules. Various results are developed, many extending known results in [1 Bennis , D. , Mahdou , N. ( 2007 ). Strongly Gorenstein projective, injective, and flat modules . J. Pure Appl. Algebra 210 : 437 – 445 .[Crossref], [Web of Science ®] , [Google Scholar]]. We also characterize FC rings in terms of strongly Gorenstein FP-injective, projective, and flat modules.     

On Some Artinian QF-3 Rings

Carl Faith in 2003 introduced and investigated an interesting class of rings over which every cyclic right module has Σ-injective injective hull (abbr., right CSI-rings) [5 Faith , C. ( 2003 ). When cyclic modules have Σ-injective hulls . Comm. Algebra 13 : 4161 – 4173 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]]. Inspired by this we investigate rings over which every cyclic right R-module has a projective Σ-injective injective hull. We show that a ring R satisfies this condition if and only if R is right artinian, the injective hull of R _R is projective and every simple right R-module is embedded in R _R . We also characterize right artinian rings in terms of injective faithful right ideals and right CSI-rings.     

Power series over strongly Hopfian bounded rings

Let R be a commutative ring with identity. We show that R[[X]] is strongly Hopfian bounded if and only if R has a strongly Hopfian bounded extension T such that I_c(T) contains a regular element of T. We deduce that if R[[X]] is strongly Hopfian bounded, then so is R[[X,Y]] where X,Y are two indeterminates over R. Also we show that if R is embeddable in a zero-dimensional strongly Hopfian bounded ring, then so is R[[X]] (this generalizes most results of Hizem [11 Hizem, S. (2011). Formal power series over strongly Hopfian rings. Commun. Algebra 39(1):279–291.[Taylor & Francis Online], [Web of Science ®] , [Google Scholar]]). For a chained ring R, we show that R[[X]] is strongly Hopfian if and only if R is strongly Hopfian.     

On quotients of generalized Euclidean group rings

Let R = ?[C] be the integral group ring of a finite cyclic group C. Dennis et al. [4 Dennis, K., Magurn, B., Vaserstein, L. (1984). Generalized Euclidean group rings. J. Reine Angew. Math. 351:113–128.[Web of Science ®] , [Google Scholar]] proved that R is a generalized Euclidean ring in the sense of Cohn [3 Cohn, P. M. (1966). On the structure of the GL₂ of a ring. Inst. Hautes Études Sci. Publ. Math. 30:5–53.[Crossref] , [Google Scholar]], i.e., SL_n(R) is generated by the elementary matrices for all n. We prove that every proper quotient of R is also a generalized Euclidean ring.     

Feebly Baer Rings and Modules

A right module M over a ring R is called feebly Baer if, whenever xa = 0 with x ∈ M and a ∈ R, there exists e² = e ∈ R such that xe = 0 and ea = a. The ring R is called feebly Baer if R_R is a feebly Baer module. These notions are motivated by the commutative analog discussed in a recent paper by Knox, Levy, McGovern, and Shapiro [6 Knox , M. L. , Levy , R. , McGovern , W. Wm. , Shapiro , J. ( 2009 ) Generalizations of complemented rings with applications to rings of functions. . J. Alg. Appl. 8 ( 1 ): 17 – 40 .[Crossref] , [Google Scholar]]. Basic properties of feebly Baer rings and modules are proved, and their connections with von Neumann regular rings are addressed.     

The Average Sylow Multiplicity Character and the Solvable Residual

Let G be a finite group, and let p₁,…, p_m be the distinct prime divisors of |G|. Given a sequence 𝒫 =P₁,…, P_m, of Sylow p_i-subgroups of G, and g ∈ G, denote by m_𝒫(g) the number of factorizations g = g₁…g_m such that g_i ∈ P_i. The properly normalized average of m_𝒫 over all 𝒫 is a complex character over G whose kernel contains the solvable radical of G [7 Levy , D. ( 2010 ). The average Sylow multiplicity character and solvability of finite groups . Communications in Algebra. 38 : 632 – 644 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]]. The present paper characterizes the solvable residual of G in terms of this character.     

Special Properties of Rings of Skew Generalized Power Series

Let R be a ring, S a strictly ordered monoid, and ω: S → End(R) a monoid homomorphism. In [30 Marks , G. , Mazurek , R. , Ziembowski , M. ( 2010 ). A unified approach to various generalizations of Armendariz rings . Bull. Aust. Math. Soc. 81 : 361 – 397 .[Crossref], [Web of Science ®] , [Google Scholar]], Marks, Mazurek, and Ziembowski study the (S, ω)-Armendariz condition on R, a generalization of the standard Armendariz condition from polynomials to skew generalized power series. Following [30 Marks , G. , Mazurek , R. , Ziembowski , M. ( 2010 ). A unified approach to various generalizations of Armendariz rings . Bull. Aust. Math. Soc. 81 : 361 – 397 .[Crossref], [Web of Science ®] , [Google Scholar]], we provide various classes of nonreduced (S, ω)-Armendariz rings, and determine radicals of the skew generalized power series ring R[[S ^≤, ω]], in terms of those of an (S, ω)-Armendariz ring R. We also obtain some characterizations for a skew generalized power series ring to be local, semilocal, clean, exchange, uniquely clean, 2-primal, or symmetric.     

Almost-Perfect Rings and Modules

Following [1 Amini , A. , Ershad , M. , Sharif , H. ( 2008 ). Rings over which flat covers of finitely generated modules are projective . Comm. Algebra 36 : 2862 – 2871 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]], a ring R is called right almost-perfect if every flat right R-module is projective relative to R. In this article, we continue the study of these rings and will find some new characterizations of them in terms of decompositions of flat modules. Also we show that a ring R is right almost-perfect if and only if every right ideal of R is a cotorsion module. Furthermore, we prove that over a right almost-perfect ring, every flat module with superfluous radical is projective. Moreover, we define almost-perfect modules and investigate some properties of them.     

Residually *-Abelian Valuation

We are given a division ring D with involution (*) and with a *-valuation V such that V(aa*b ? baa*) >₀ V(aa*b), for all nonzero elements a, b of D. We assume, further, that the associated residue class division ring D _V is a (commutative) field with characteristic 0. In this work, we evidence two criteria in order for D to be either a standard quaternion division algebra or else a purely transcendental extension of its center. We apply one of these to answer our open Question 2.4.3 [2 Chacron , M. ( 2012 ). On *-semiordering . Comm. Algebra 40 : 3783 – 3791 .[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]].     

The McCoy Condition on Ore Extensions

Nielsen [29 Nielsen , P. P. ( 2006 ). Semi-commutativity and the McCoy condition . J. Algebra 298 : 134 – 141 .[Crossref], [Web of Science ®] , [Google Scholar]] proved that all reversible rings are McCoy and gave an example of a semicommutative ring that is not right McCoy. When R is a reversible ring with an (α, δ)-condition, namely (α, δ)-compatibility, we observe that R satisfies a McCoy-type property, in the context of Ore extension R[x; α, δ], and provide rich classes of reversible (semicommutative) (α, δ)-compatible rings. It is also shown that semicommutative α-compatible rings are linearly α-skew McCoy and that linearly α-skew McCoy rings are Dedekind finite. Moreover, several extensions of skew McCoy rings and the zip property of these rings are studied.     

Commutative rings with genus two annihilator graphs

Let R be a commutative ring and Z(R)* be its set of all nonzero zero-divisors. The annihilator graph of a commutative ring R is the simple undirected graph AG(R) with vertices Z(R)*, and two distinct vertices x and y are adjacent if and only if ann(xy)≠ann(x)∪ann(y). The notion of annihilator graph has been introduced and studied by Badawi [8 Badawi, A. (2014). On the annihilator graph of a commutative ring. Commun. Algebra 42(1):108–121.[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]]. In this paper, we classify the finite commutative rings whose AG(R) are projective. Also we determine all isomorphism classes of finite commutative rings with identity whose AG(R) has genus two.