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In this paper, we study supplement submodules with respect to a torsion class of any torsion theory, generalizing the concepts of sa-supplement, extended S-supplement, Rad-supplement submodules, etc. We give some homological properties of this class of modules, which forms a proper class, and use them to characterize some rings in terms of those certain modules. [ABSTRACT FROM AUTHOR]

This is a survey of some of the consequences of the recently introduced congruences on the theory of connectednesses (radical classes) and disconnectednesses (semisimple classes) of graphs and topological spaces. In particular, it is shown that the connectednesses and disconnectednesses can be obtained as Hoehnke radicals and a connectedness has a characterization in terms of congruences resembling the classical characterization of its algebraic counterpart using ideals for a radical class. But this approach has also shown that there are some unexpected differences and surprises: an ideal-hereditary Hoehnke radical of topological spaces or graphs need not be a Kurosh-Amitsur radical and in the category of graphs with no loops, non-trivial connectednesses and disconnectednesses exist, but all Hoehnke radicals degenerate. [ABSTRACT FROM AUTHOR]

In this manuscript, we define when a torsion theory, which is a generalization of Goldie's torsion theory, is of the type I, II, or III, according to Kaplansky's theory of types, and we establish some structure theorems of regular, right self-injective rings using the torsion theories of Kaplansky-type. We study the properties that each of these torsion theories has and their influence on the lattice [ τ g , χ ] of generalizations of Goldie's torsion theory, locating several subintervals of [ τ g , χ ] which contain no atoms nor coatoms, thus extending our knowledge of the lattice structure of [ τ g , χ ] . [ABSTRACT FROM AUTHOR]

For a commutative, unital and integral quantale V, we generalize to V-groups the results developed by Gran and Michel for preordered groups. We first of all show that, in the category V- Grp of V-groups, there exists a torsion theory whose torsion and torsion-free subcategories are given by those of indiscrete and separated V-groups, respectively. It turns out that this torsion theory induces a monotone-light factorization system that we characterize, and it is then possible to describe the coverings in V- Grp . We next classify these coverings as internal actions of a Galois groupoid. Finally, we observe that the subcategory of separated V-groups is also a torsion-free subcategory for a pretorsion theory whose torsion subcategory is the one of symmetric V-groups. As recently proved by Clementino and Montoli, this latter category is actually not only coreflective, as it is the case for any torsion subcategory, but also reflective. [ABSTRACT FROM AUTHOR]

A generalized torsion theory associated with a Serre subcategory was introduced to characterize the Melkersson condition. However, little is known about when generalized torsion theories can be realized using specialization closed subsets of the spectrum of a ring. In this article, we investigate the characterization of generalized torsion theories realized by such a subset. [ABSTRACT FROM AUTHOR]

Let A be a complete and cocomplete abelian category with the additional assumption that any direct sum and product of short exact sequences are exact. We explore the relation between balanced pairs and TTF triples in A. The main results are: (1) every balanced pair in A induces a TTF triple; (2) if A has projective covers and injective envelopes, then every TTF triple in A gives rise to a balanced pair, and hence there is a bijection between the equivalence classes of balanced pairs and TTF triples in A ; (3) a balanced pair in A is quasi admissible if and only if its induced TTF triple is centrally splitting. Our first application of these results provide abundant rings over which every balanced pair is quasi admissible, including local rings, commutative semiperfect rings, and commutative Noetherian rings. Another application is the classification of equivalence classes of cohereditary balanced pairs over arbitrary rings. We also present counterexamples to [15, Open questions 3 and 5]. Finally, we prove that the answers to [15, Open questions 2 and 4] are positive for coherent rings with weak global dimension at most one. [ABSTRACT FROM AUTHOR]

In this work we introduce a new concept, namely, purely τ s-extending modules (rings) which is torsion-theoretic analogues of extending modules and then we extend many results from extending modules to this new concept. For instance, we show that for any ring R with unit, RR is purely τ s-extending if and only if every cyclic τ-nonsingular R-module is flat. Also, we make a classification for the direct sums of the rings to be purely τ s-extending. [ABSTRACT FROM AUTHOR]

We explore a non-abelian torsion theory in the category of preordered groups: the objects of its torsion-free subcategory are the partially ordered groups, whereas the objects of the torsion subcategory are groups (with the total order). The reflector from the category of preordered groups to this torsion-free subcategory has stable units, and we prove that it induces a monotone-light factorization system. We describe the coverings relative to the Galois structure naturally associated with this reflector, and explain how these coverings can be classified as internal actions of a Galois groupoid. Finally, we prove that in the category of preordered groups there is also a pretorsion theory, whose torsion subcategory can be identified with a category of internal groups. This latter is precisely the subcategory of protomodular objects in the category of preordered groups, as recently discovered by Clementino, Martins-Ferreira, and Montoli. [ABSTRACT FROM AUTHOR]

We give a list of statements on the geometry of elliptic threefolds phrased only in the language of topology and homological algebra. Using only notions from topology and homological algebra, we recover existing results and prove new results on torsion pairs in the category of coherent sheaves on an elliptic threefold. [ABSTRACT FROM AUTHOR]

Using congruences, a Hoehnke radical can be defined for graphs in the same way as for universal algebras. This leads in a natural way to the connectednesses and disconnectednesses (= radical and semisimple classes) of graphs. It thus makes sense to talk about ideal-hereditary Hoehnke radicals for graphs (= hereditary torsion theories). All such radicals for the category of undirected graphs which allow loops are explicitly determined. Moreover, in contrast to what is the case for the well-known algebraic categories, it is shown here that such radicals for graphs need not be Kurosh–Amitsur radicals. [ABSTRACT FROM AUTHOR]

This paper is the first one in a series of three dealing with the concept of injective stabilization of the tensor product and its applications. Its primary goal is to collect known facts and establish a basic operational calculus that will be used in the subsequent parts. This is done in greater generality than is necessary for the stated goal. Several results of independent interest are also established. They include, among other things, connections with satellites, an explicit construction of the stabilization of a finitely presented functor, various exactness properties of the injectively stable functors, a construction, from a functor and a short exact sequence, of a doubly-infinite exact sequence by splicing the injective stabilization of the functor and its derived functors. When specialized to the tensor product with a finitely presented module, the injective stabilization with coefficients in the ring is isomorphic to the 1-torsion functor. The Auslander-Reiten formula is extended to a more general formula, which holds for arbitrary (i.e., not necessarily finite) modules over arbitrary associative rings with identity. Weakening of the assumptions in the theorems of Eilenberg and Watts leads to characterizations of the requisite zeroth derived functors. The subsequent papers, provide applications of the developed techniques. Part II deals with new notions of torsion module and cotorsion module of a module. This is done for arbitrary modules over arbitrary rings. Part III introduces a new concept, called the asymptotic stabilization of the tensor product. The result is closely related to different variants of stable homology (these are generalizations of Tate homology to arbitrary rings). A comparison transformation from Vogel homology to the asymptotic stabilization of the tensor product is constructed and shown to be epic. [ABSTRACT FROM AUTHOR]

We give a classification of torsion classes (or nullity classes) in an abelian category by forming a spectrum of equivalence classes of premonoform objects. This is parallel to Kanda's classification of Serre subcategories. [ABSTRACT FROM AUTHOR]

This paper deals with an order-theoretic analysis of certain structures studied in category theory. A categorical closure operator (cco in short) is a structure on a category, which mimics the structure on the category of topological spaces formed by closing subspaces of topological spaces. Such structures play a significant role not only in categorical topology, but also in topos theory and categorical algebra. In the case when the category is a poset, as a particular instance of the notion of a cco, one obtains what we call in this paper a binary closure operator (bco in short). We show in this paper that bco's allow one to see more easily the connections between standard conditions on general cco's, and furthermore, we show that these connections for cco's can be even deduced from the corresponding ones for bco's, when considering cco's relative to a well-behaved class of monorphisms as in the literature. The main advantage of the approach to such cco's via bco's is that the notion of a bco is self-dual (relative to the usual posetal duality), and by applying this duality to cco's, independent results on cco's are brought together. In particular, we can unify basic facts about hereditary closure operators with similar facts about minimal closure operators. Bco's also reveal some new links between categorical closure operators, the usual unary closure and interior operators, modularity law in order and lattice theory, the theory of factorization systems and torsion theory. [ABSTRACT FROM AUTHOR]

A Galois connection between functorial topologies on abelian groups and subclasses of abelian groups is constructed by means of the notion of indiscrete topology. It is shown that the composition of this Galois connection with a previously introduced one coincides with the classical Galois connection induced by the notion of constant morphism. Examples are provided. [ABSTRACT FROM AUTHOR]

In this paper, which is a continuation of our previous paper [T. Albu, M. Iosif, A. Tercan, The conditions ( C) in modular lattices, and applications, J. Algebra Appl. 15 (2016), http: dx.doi.org/10.1142/S0219498816500018], we investigate the latticial counterparts of some results about modules satisfying the conditions ( C) or ( C). Applications are given to Grothendieck categories and module categories equipped with hereditary torsion theories. [ABSTRACT FROM AUTHOR]

The purpose of this article is to prove that the category of cocommutative Hopf K-algebras, over a field K of characteristic zero, is a semi-abelian category. Moreover, we show that this category is action representable, and that it contains a torsion theory whose torsion-free and torsion parts are given by the category of groups and by the category of Lie K-algebras, respectively. [ABSTRACT FROM AUTHOR]

A semi-localization of a category is a full reflective subcategory with the property that the reflector is semi-left-exact. There are many interesting examples of semi-localizations, as for instance any torsion-free subcategory of a semi-abelian category. By specializing a result due to S. Mantovani, we first characterize the categories which are semi-localizations of exact Mal'tsev categories. We then prove a new characterization of protomodular categories in terms of binary relations, allowing us to obtain an abstract characterization of the semi-localizations of exact protomodular categories. This result is very useful to study the (hereditarily)-torsion-free subcategories of semi-abelian categories. Some examples are considered in detail in the categories of groups, crossed modules, commutative rings and topological groups. We finally explain how these results extend the corresponding ones obtained in the abelian context by W. Rump. [ABSTRACT FROM AUTHOR]

We characterize t-structures in stable ∞-categories as suitable quasicategorical factorization systems. More precisely we show that a t-structure 픱 on a stable ∞-category C is equivalent to a normal torsion theory 픽 on C, i.e. to a factorization system 픽 = (퓔, ℳ) where both classes satisfy the 3-for-2 cancellation property, and a certain compatibility with pullbacks/pushouts. [ABSTRACT FROM AUTHOR]

In this paper, we introduce and investigate the latticial counterparts of the conditions (Ci), i = 1, 2, 3, 11, 12, for modules. In particular, we study the lattices satisfying the condition (C1), we call CC lattices (for Closed are Complements), i.e. the lattices such that any closed element is a complement, that are the latticial counterparts of CS modules (for Closed are Summands). Applications of these results are given to Grothendieck categories and module categories equipped with a torsion theory. [ABSTRACT FROM AUTHOR]

In this paper we introduce and investigate the latticial counterpart of the module-theoretical concept of preradical. Applications are given to Grothendieck categories and module categories equipped with hereditary torsion theories. [ABSTRACT FROM AUTHOR]

In this paper, we study module theoretic definitions of the Baer and related ring concepts. We say a module is s.Baer if the right annihilator of a nonempty subset of the module is generated by an idempotent in the ring. We show that s.Baer modules satisfy a number of closure properties. Under certain conditions, a torsion theory is established for the s.Baer modules, and we provide examples of s.Baer torsion modules and modules with a nonzero s.Baer radical. The other principal interest of this paper is to provide explicit connections between s.Baer modules and projective modules. Among other results, we show that every s.Baer module is an essential extension of a projective module. Additionally, we prove, with limited and natural assumptions, that in a generalized triangular matrix ring every s.Baer submodule of the ring is projective. As an application, we show that every prime ring with a minimal right ideal has the strong summand intersection property. Numerous examples are provided to illustrate, motivate, and delimit the theory. [ABSTRACT FROM AUTHOR]

We first characterize τ-complemented modules with relative (pre)covers. We also introduce an extending module relative to τ-pure submodules on a hereditary torsion theory τ and give its relationship with τ-complemented modules. [ABSTRACT FROM AUTHOR]

Using the concept of prime submodule forM ∈ R-Mod,P ∈ Spec(M), andN ∈ σ[M], we define whenNisP-Mtame\ (Mtame) module. This concept generalizes the concept\ ofP-tame(tame) modules. ForM ∈ R-Mod and τ ∈M-tors, we use the concept of τM-Gabriel dimension and we study the relationship betweenMtamemodules and τM-Gabriel dimension. We find equivalent conditions for a moduleMprogenerator in σ[M] with τM-Gabriel dimension to have τM-Gabriel correspondence in terms of theP-Mtamemodules. This result extends the results by Albu et al. and Kim and Krause. [ABSTRACT FROM AUTHOR]

We formally introduce and study generalized comatrix coalgebras and upper triangular comatrix coalgebras, which are not only a dualization but also an extension of classical generalized matrix algebras. We use these to answer several open questions on Noetherian and Artinian type notions in the theory of coalgebras, and to give complete connections between these. We also solve completely the so called finite splitting problem for coalgebras: we show thatis a coalgebra such that the rational part of every left finitely generated-module splits off if and only ifis an upper triangular matrix coalgebra, for a serial coalgebrawhose Ext-quiver is a finite union of cycles, a finite dimensional coalgebraand a finite dimensional bicomodule. [ABSTRACT FROM AUTHOR]

Using the nice properties of thew-divisible weight and thew-divisible groups, we prove a factorization theorem for compact abelian groupsK; namely,K = Ktor × Kd, whereKtoris a bounded torsion compact abelian group andKdis aw-divisible compact abelian group. By Pontryagin duality this result is equivalent to the same factorization for discrete abelian groups proved in [9]. [ABSTRACT FROM AUTHOR]

Using the concept of prime submodule defined by Raggi et al. in [16], forM ∈ R-Mod we define the concept of classical Krull dimension relative to a hereditary torsion theory τ ∈M-tors. We prove that ifMis progenerator in σ[M], τ ∈M-tors such thatMhas τ-Krull dimension thencl.Kτdim (M) ≤ kτ(M). Also we show that ifMis noetherian, τ-fully bounded, progenerator of σ[M], andM∈ 𝔽τ, thencl·Kτdim (M) = kτ(M). [ABSTRACT FROM AUTHOR]

Abstract: Bijective correspondences are established between endofinite injective left modules, endofinite flat right modules, finite collections of minimal noetherian prime ideals, normalized rank functions on left ideals and characters. Endofinite flat modules are identified as flat covers of modules associated to a minimal noetherian prime ideal, while endofinite flat injectives are characterized by localizations with a semiprimary QF-3 quotient ring. [Copyright &y& Elsevier]

This is the second part of the paper with the same title published inCommunications in Algebrain 2011. It contains applications of the Latticial Osofsky–Smith Theorem to Grothendieck categories and module categories equipped with a torsion theory. Various many different meanings spread in the literature of the relative concepts with respect to a hereditary torsion theory τ on Mod-Rlike τ-essential submodule, τ-complement submodule, τ-CS module, etc. are also discussed. [ABSTRACT FROM PUBLISHER]

We show that every two-term tilting complex over an Artin algebra has a tilting module over a certain factor algebra as a homology group. Also, we determine the endomorphism algebra of such a homology group, which is given as a certain factor algebra of the endomorphism algebra of the two-term tilting complex. Thus, every derived equivalence between Artin algebras given by a two-term tilting complex induces a derived equivalence between the corresponding factor algebras. [ABSTRACT FROM PUBLISHER]

Abstract: Given a torsion theory in an abelian category , the reflector to the torsion-free subcategory induces a reflective factorisation system on . It was shown by A. Carboni, G.M. Kelly, G. Janelidze and R. Paré that induces a monotone-light factorisation system by simultaneously stabilising and localising , whenever the torsion theory is hereditary and any object in is a quotient of an object in . We extend this result to arbitrary normal categories, and improve it also in the abelian case, where the heredity assumption on the torsion theory turns out to be redundant. Several new examples of torsion theories where this result applies are then considered in the categories of abelian groups, groups, topological groups, commutative rings, and crossed modules. [Copyright &y& Elsevier]

In this article we define and compare different types of the notion of grade with respect to torsion theories over commutative rings which are not necessarily Noetherian. We do this by using Ext-modules, Koszul cohomology modules, and Čech and local cohomology modules. An application of these results is given. [ABSTRACT FROM PUBLISHER]

We study the existence of covers and envelopes by some special functors on the category of finitely presented modules. As an application, we characterize some important rings using these functors. We also investigate homological properties of some functors on the stable module category. The relationship between phantoms and Ext-phantoms is obtained. It is shown that every leftR-moduleMhas an Ext-phantom preenvelopef:M → Nwith coker(f) pure-projective. Finally, we prove that, as a torsionfree class of (mod-R, Ab), (mod-R, Ab) is generated by theFP-injective objects. [ABSTRACT FROM AUTHOR]

We consider the concept of prime submodule defined by Raggi et al. [7]. We find equivalent conditions for a module M progenerator in σ[M], with τ M -Gabriel dimension, to have a one-to-one correspondence between the set of isomorphism classes of indecomposable τ-torsion free injective modules in σ[M] and the set of τ-pure submodules prime in M, where τ is a hereditary torsion theory in σ[M]. Also we give a relation between the concept of prime M-ideal given by Beachy and the concept of prime submodule in M. We obtain that if M is progenerator in σ[M], then these concepts are equivalent. [ABSTRACT FROM PUBLISHER]

Let R be a right perfect ring, and let (ℱ, 𝒞) be a cotorsion theory in the category of right R-modules ℳ R . In this article, it is shown that every right R-module has a superfluous ℱ-cover if and only if there exists a torsion theory (𝒜, ℬ) such that (ℱ, 𝒞) is cogenerated by ℬ. It is also proved that if (𝒜, ℬ) is a cosplitting torsion theory, then (⊥ℬ, (⊥ℬ)⊥) is a hereditary and complete cotorsion theory, and if (𝒜, ℬ) is a centrally splitting torsion theory, then (⊥ℬ, (⊥ℬ)⊥) is a hereditary and perfect cotorsion theory. [ABSTRACT FROM AUTHOR]

Let R be a ring. A collection of R-modules containing the zero module and closed under isomorphisms will be denoted by X. An R-module M is said to be X -lifting if for every X-submodule N of M there exists A ≤ N such that M = A ⊕ B and N ∩ B is small in B [11]. In the present paper, we consider the question: Can we characterize X-lifting modules via objects of the class X ? [ABSTRACT FROM AUTHOR]

Let R be a ring and let A be a hereditary torsion class of R-modules. The inclusion of the localizing subcategory generated by A into the derived category of R has a right adjoint, denoted CellA. Recently, Benson has shown how to compute CellA R when R is a group ring of a finite group over a prime field and A is the hereditary torsion class generated by a simple module. We generalize Benson's construction to the case where A is any hereditary torsion class on R. It is shown that for every R-module M there exists an injective R-module E such that: Hn (CellAM) ≅ Ext Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. (Hom R(M,E),E) for n ≥ 2. [ABSTRACT FROM AUTHOR]

We consider a generalization of lifting modules relative to a class $\mathcal{A}$ of modules and a proper class 피 of short exact sequences of modules. These modules will be called 피-$\mathcal{A}$-lifting. We establish characterizations of modules with the property that every direct sum of copies of them is 피-$\mathcal{A}$-lifting. [ABSTRACT FROM AUTHOR]

An R-module M is said to be a C1-module if every closed submodule of M is a direct summand. In this paper we introduce and investigate the concept of the τ-C1 module for a hereditary torsion theory τ on Mod-R. τ -C1 modules are a generalization of C1-modules. [ABSTRACT FROM AUTHOR]

Let R be a ring. A right R-module U is called Tor-tilting if [image omitted], where U+ = Hom(U, /), Cogen(U+) is the class of left R-modules cogenerated by U+ and [image omitted] consists of modules RM such that [image omitted]. Some examples and characterizations of Tor-tilting modules are given. Among others, it is shown that UR is Tor-tilting if and only if U+ is cotilting. Moreover, both tilting modules and completely faithful flat modules are proved to be Tor-tilting. The torsion theory induced by a Tor-tilting module is also investigated. [ABSTRACT FROM AUTHOR]

Perfect Gabriel filters of right ideals and their corresponding right rings of quotients have the desirable feature that every module of quotients is determined solely by the right ring of quotients. On the other hand, symmetric rings of quotients have a symmetry that mimics the commutative case. In this paper, we study rings of quotients that combine these two desirable properties. We define the symmetric versions of a right perfect ring of quotients and a right perfect Gabriel filter — the perfect symmetric ring of quotients and the perfect symmetric Gabriel filter and study their properties. Then we prove that the standard construction of the total right ring of quotients $Q^r_{\rm tot}(R)$ can be adapted to the construction of the largest perfect symmetric ring of quotients — the total symmetric ring of quotients $Q^\sigma_{\rm tot}(R)$. We also demonstrate that Morita's construction of $Q^r_{\rm tot}(R)$ can be adapted to the construction of $Q^\sigma_{\rm tot}(R)$. [ABSTRACT FROM AUTHOR]

Abstract: A Grothendieck category is said to be locally finitely generated if the subobject lattice of every object in is compactly generated, or equivalently, if possesses a family of finitely generated generators. Every nonzero locally finitely generated Grothendieck category possesses simple objects. We shall call a Grothendieck category indecomposable if is not equivalent to a product of nonzero Grothendieck categories . In this paper an example of an indecomposable nonlocally finitely generated Grothendieck category possessing simple objects is constructed, answering in the negative a sharper form of a question posed by Albu, Iosif, and Teply in [T. Albu, M. Iosif, M.L. Teply, Dual Krull dimension and quotient finite dimensionality, J. Algebra 284 (2005) 52–79]. [Copyright &y& Elsevier]

We define and study the symmetric version of differential torsion theories. We prove that the symmetric versions of some of the existing results on derivations on right modules of quotients hold for derivations on symmetric modules of quotients. In particular, we prove that the symmetric Lambek, Goldie, and perfect torsion theories are differential. We also study conditions under which a derivation on a right or symmetric module of quotients extends to a right or symmetric module of quotients with respect to a larger torsion theory. Using these results, we study extensions of ring derivations to maximal, total, and perfect right and symmetric rings of quotients. [ABSTRACT FROM AUTHOR]

Abstract: Let A be a pseudocompact (or profinite) algebra, so where C is a coalgebra. We show that the if the semiartinian part (the “Dickson” part) of every A-module M splits off in M, then A is semiartinian, giving thus a positive answer in the case of algebras arising as dual of coalgebras (pseudocompact algebras), to a well known conjecture of Faith. [Copyright &y& Elsevier]

Given a hereditary torsion theory τ = (T,F) in Mod-R, a module M is called τ-supplemented if every submodule A of M contains a direct summand C of M with A/C τ-torsion. A submodule V of M is called τ - supplement of U in M if U + V = M and U ∩ V ≤(V) and M is τ - weakly supplemented if every submodule of M has a τ-supplement in M. Let M be a τ-weakly supplemented module. Then M has a decomposition M = M1 ⊕ M2 where M1 is a semisimple module and M2 is a module with τ(M2) ≤e M2. Also, it is shown that; any finite sum of τ-weakly supplemented modules is a τ-weakly supplemented module. [ABSTRACT FROM AUTHOR]

Abstract: In [R. Colpi, K.R. Fuller, Tilting objects in abelian categories and quasitilted rings, Trans. Amer. Math. Soc., in press] tilting objects in an arbitrary abelian category are introduced and are shown to yield a version of the classical tilting theorem between and the category of modules over their endomorphism rings. Moreover, it is shown that given any faithful torsion theory in , for a ring R, the corresponding Heart is an abelian category admitting a tilting object which yields a tilting theorem between the Heart and . In this paper we first prove that is a prototype for any abelian category admitting a tilting object which tilts to in . Then we study AB-type properties of the Heart and commutations with direct limits. This allows us to show, for instance, that any abelian category with a tilting object is AB4, and to find necessary and sufficient conditions which guarantee that is a Grothendieck or even a module category. As particular situations, we examine two main cases: when is hereditary cotilting, proving that is Grothendieck and when is tilting, proving that is a module category. [Copyright &y& Elsevier]

We show that if A is a representation-finite selfinjective Artin algebra, then every P• ∈ Kb(&Pscr;A) with HomK(Mod-A)(P•,P•[i]) = 0 for i ≠ 0 and add(P•) = add(νP•) is a direct summand of a tilting complex, and that if A, B are derived equivalent representation-finite selfinjective Artin algebras, then there exists a sequence of selfinjective Artin algebras A = B0, B1,.., Bm = B such that, for any 0 ≤ i < m, Bi+1 is the endomorphism algebra of a tilting complex for Bi of length ≤ 1. [ABSTRACT FROM AUTHOR]

Abstract: Many known results on finite von Neumann algebras are generalized, by purely algebraic proofs, to a certain class of finite Baer *-rings. The results in this paper can also be viewed as a study of the properties of Baer *-rings in the class . First, we show that a finitely generated module over a ring from the class splits as a direct sum of a finitely generated projective module and a certain torsion module. Then, we define the dimension of any module over a ring from and prove that this dimension has all the nice properties of the dimension studied in [W. Lück, J. Reine Angew. Math. 495 (1998) 135–162] for finite von Neumann algebras. This dimension defines a torsion theory that we prove to be equal to the Goldie and Lambek torsion theories. Moreover, every finitely generated module splits in this torsion theory. If R is a ring in , we can embed it in a canonical way into a regular ring Q also in . We show that is isomorphic to by producing an explicit isomorphism and its inverse of monoids that extends to the isomorphism of and . [Copyright &y& Elsevier]

For an ordinal α, a modular lattice L with 0 and 1 is α-atomic if L has dual Krull dimension α but each interval [0,x] with x < 1 has dual Krull dimension <α. The properties of α-atomic lattices are presented and applied to module theory. The endomorphism ring of certain types of α-atomic modules is a local domain and hence there is a Krull–Schmidt type theorem for those α-atomic modules. [ABSTRACT FROM AUTHOR]

We show that, if E is a commutative MU-algebra spectrum such that E* is Landweber exact over MU*, then the category of E*E-comodules is equivalent to a localization of the category of MU*MU-comodules. This localization depends only on the heights of E at the integer primes p. It follows, for example, that the category of E(n)*E(n)-comodules is equivalent to the category of (vn-1BP)*(vn-1BP)-comodules. These equivalences give simple proofs and generalizations of the Miller–Ravenel and Morava change of rings theorems. We also deduce structural results about the category of E*E-comodules. We prove that every E*E-comodule has a primitive, we give a classification of invariant prime ideals in E*, and we give a version of the Landweber filtration theorem. [Copyright &y& Elsevier]

Let R be a ring, M a right R-module and a hereditary torsion theory in Mod-R with associated torsion functor τ for the ring R. Then M is called τ-supplemented when for every submodule N of M there exists a direct summand K of M such that K ≤ N and N/K is τ-torsion module. In [4], M is called almost τ-torsion if every proper submodule of M is τ-torsion. We present here some properties of these classes of modules and look for answers to the following questions posed by the referee of the paper [4]: (1) Let a module M = M′ ⊕ M″ be a direct sum of a semisimple module M′ and τ-supplemented module M″. Is M τ-supplemented? (2) Can one find a non-stable hereditary torsion theory τ and τ-supplemented modules M′ and M″ such that M′ ⊕ M″ is not τ-supplemented? (3) Can one find a stable hereditary torsion theory τ and a τ-supplemented module M such that M/N is not τ-supplemented for some submodule N of M? (4) Let τ be a non-stable hereditary torsion theory and the module M be a finite direct sum of almost τ-torsion submodules. Is M τ-supplemented? (5) Do you know an example of a torsion theory τ and a τ-supplemented module M with τ-torsion submodule τ(M) such that M/τ(M) is not semisimple? [ABSTRACT FROM AUTHOR]