Indecomposable division algebras with a baer ordering

In this paper we construct Baer ordered indecomposable division algebras of index p ⁿ and exponent p ^m for all primes p and for all n m, with n>m≥1 (n≥3 if p=2).     

Generalised nilpotence conditions in n-engel lie algebras

A Lie algebra is said to satisfy the Baer condition provided each of its 1-dimensional subalgebras is a subideal; if the defect is bounded then the Lie algebra is bounded Baer. We first characterise restricted enveloping algebras (of odd characteristic p) that satisfy the bounded Baer condition. Using this characterisation, we are then able to construct a Lie algebra satisfying the bounded Engel condition, in each odd characteristic p, that does not satisfy the Baer condition. Such a bounded Engel Lie algebra must contain a non-nilpotent 1-generated ideal     

Decomposability and embeddability of discretely Henselian division algebras

LetF be a discretely Henselian field of rank one, with residue fieldk a number field, and letD/F be anF-division algebra. We conduct an exhaustive study of the decomposability of an arbitraryD. Specifically, we prove the following:D has a semiramified (SR)F-division subalgebra if and only ifD has a totally ramified (TR) subfield. However, there may be TR subfields not contained in any SR subalgebra. IfD has prime-power index, thenD is decomposable if and only ifD properly contains a SR division subalgebra. Equivalently,D has a decomposable Sylow factor if and only if ii(D ^⊗n )≠1/n i(D) for somen dividing the period ofD, that is, if and only if the index fails to mimic the behavior of the period ofD. There exists indecomposableD with prime-power periodp ² and indexp ³. Every proper division subalgebra ofD is indecomposable. Conversely, every indecomposableF-division algebra ofp-power index embeds properly in someD ofp-power index if and only ifk does not have a certain strengthened form of class field theory’s Special Case. Semiramified division algebras and division algebras of odd index always properly embed. Finally, these results apply to an extent overk(t), and we prove that there exist indecomposablek(t)-division algebras of periodp ² and indexp ³, solving an open problem of Saltman. Dedicated to the memory of Amitsur Research supported in part by NSF Grant DMS-9100148.     

Non-crossed products over function fields

Noncrossed product division algebras are constructed over all function fields and iterated power series fields over global fields, using Hilbert's Irreducibility Theorem and the construction of [B]. Minimum indexes obtained are p ² for odd p and 2³ otherwise. Examples are obtained with large index to exponent ratio. Received: 12 February 2001 / Revised version: 26 November 2001     

Baer and Baer *-ring characterizations of Leavitt path algebras

We characterize Leavitt path algebras which are Rickart, Baer, and Baer ?-rings in terms of the properties of the underlying graph. In order to treat non-unital Leavitt path algebras as well, we generalize these annihilator-related properties to locally unital rings and provide a more general characterizations of Leavitt path algebras which are locally Rickart, locally Baer, and locally Baer ?-rings. Leavitt path algebras are also graded rings and we formulate the graded versions of these annihilator-related properties and characterize Leavitt path algebras having those properties as well.Our characterizations provide a quick way to generate a wide variety of examples of rings. For example, creating a Baer and not a Baer ?-ring, a Rickart ?-ring which is not Baer, or a Baer and not a Rickart ?-ring, is straightforward using the graph-theoretic properties from our results. In addition, our characterizations showcase more properties which distinguish behavior of Leavitt path algebras from their $C^{?}$-algebra counterparts. For example, while a graph $C^{?}$-algebra is Baer (and a Baer ?-ring) if and only if the underlying graph is finite and acyclic, a Leavitt path algebra is Baer if and only if the graph is finite and no cycle has an exit, and it is a Baer ?-ring if and only if the graph is a finite disjoint union of graphs which are finite and acyclic or loops.     

Primitively generated Hopf orders in characteristic p

Let R be a characteristic p discrete valuation ring with field of fractions K. Let H be a commutative, cocommutative K-Hopf algebra of p-power rank which is generated as a K-algebra by primitive elements. We construct all of the R-Hopf orders of H in K; each Hopf order corresponds to a solution to a single matrix equation. For R complete, we greatly simplify the matrix equation and give explicit examples of Hopf orders in some rank p² K-Hopf algebras.     

Nondegenerate Semiramified Valued and Graded Division Algebras

The main goal of this article is to give examples of division p-algebras that are not tensor product of cyclic algebras (Corollary 2.19) and to prove that nondegenerate tame semiramified division algebras of prime power degree over a Henselian valued field are indecomposable (Theorem 3.5). For this, we give new results concerning nicely semiramified division algebras over Henselian valued fields, and we develop a new study for nondegenerate valued and graded division algebras.     

Breuil–Kisin Modules and Hopf Orders in Cyclic Group Rings

For K, a finite extension of ? _p with ring of integers R, we show how Breuil–Kisin modules can be used to determine Hopf orders in K-Hopf algebras of p-power dimension. We find all cyclic Breuil–Kisin modules and use them to compute all of the Hopf orders in the group ring KΓ where Γ is cyclic of order p or p ². We also give a Laurent series interpretation of the Breuil–Kisin modules that give these Hopf orders.     

Classes of structurable algebras of skew-rank

Let R be a ring such that 2, 3 ∈ R ^×. We construct classes of structurable algebras over R whose residue class algebras have skew-dimension 1. These are matrix algebras or forms of matrix algebras which do not necessarily arise out of separable Jordan algebras of degree 3. As an application, we give canonical examples of structurable algebras of large dimension.     

Special forms of two symbols are division algebras

 If F is a valued field of characteristic p certain tensor products of cyclic F-algebras are called special forms. It is known that if F is maximally complete then every Brauer class of exponent p is represented by a special form. It is shown here that special forms of two symbols are division algebras, but that special forms of three symbols need not be, and can represent indecomposible division algebras of index $p^2$. Received: 16 October 2001     

On the Classification of Commutative Right-Nilalgebras of Nilindex 5 and Dimension 4

Let D be a division ring with center k, and let D ^? be its multiplicative group. We investigate the existence of free groups in D ^?, and free algebras and free group algebras in D. We also go through the case when D has an involution * and consider the existence of free symmetric and unitary pairs in D ^?.     

Finite dimensional Nichols algebras over certain p-groups

We investigate pointed Hopf algebras over finite nilpotent groups of odd order, with nilpotency class 2. For such a group G, we show that if its commutator subgroup coincides with its center, then there exists no non-trivial finite-dimensional pointed Hopf algebra with kG as its coradical. We apply these results to non-abelian groups of order p³, p⁴ and p⁵, and list all the pointed Hopf algebras of order p⁶, whose group of grouplikes is non-abelian.     

On Skew Inverse Laurent-Serieswise Armendariz Rings

We study the skew inverse Laurent-serieswise Armendariz (or simply, SIL-Armendariz) condition on R, a generalization of the standard Armendariz condition from polynomials to skew inverse Laurent series. We study relations between the set of annihilators in R and the set of annihilators in R((x ^?1; α)). Among applications, we show that a number of interesting properties of a SIL-Armendariz ring R such as the Baer and the α-quasi Baer property transfer to its skew inverse Laurent series extensions R((x ^?1; α)) and vice versa. For an α-weakly rigid ring R, R((x ^?1; α)) is a left p.q.-Baer ring if and only if R is left p.q.-Baer and every countable subset of S _?(R) has a generalized countable join in R. Various types of examples of SIL-Armendariz rings is provided.     

Graded Clifford Algebras of Prime Global Dimension with an Action of H p

In this article, we study graded Clifford algebras with a gradation preserving action of automorphisms given by H _p , the Heisenberg group of order p ³ with p prime. After reviewing results in dimensions 3 and 4, we will determine the graded Clifford algebras that are AS-regular algebras of global dimension 5 and generalize certain results to arbitrary dimension p.     

On the connection between the representation type of an algebra and its lattice of preradicals

In this paper, we study lattices of preradicals which are not small classes (in which case we say that the corresponding rings are p-large), and specially we consider some infinite representation type algebras. We construct an injective assignment between lattices of preradicals, using a full functor between the corresponding categories of modules, that satisfies certain conditions. We show that the polynomial ring over any field is p-large, and we use this fact to provide examples and some classes of algebras (both tame and wild) which are p-large.     

Square-free rings

Square-free algebras have been studied in a variety of settings amd have been completely characterized by Anderson and D ’Ambrosia. In this paper, we extend the definition of square-free to Artinian rings with identity and begin to develop the theory of square-free rings. We show that every square-free ring has an associated division ring and square-free semigroup. Many square-free rings are square-free D -algebras, i.e. rings of the form D? _K A where D is a division ring with center K and A is a square-free K-algebra. We present a characteriza-tion of all square-free D -algebras and provide examples of square-free rings that are not D-algebras.     

Torsion in CH2 of Severi-Brauer varieties and indecomposability of generic algebras

We compute degrees of algebraic cycles on certain Severi-Brauer varieties and apply it to show that:

Baer Criterion for Injectivity of Projection Algebras

Projection algebras are M-sets for the monoid M = (ℕ^∞, min) which are used by computer scientists for algebraic specification of process algebras. In contrast to the case of modules it is well known that the Baer Criterion does not generally hold for injectivity of M-sets for an arbitrary monoid M. Here, introducing a closure operator, we prove that the Baer Criterion does hold for injectivity of projection algebras.     

Nilpotent n-Lie Algebras

We find examples of nilpotent n-Lie algebras and prove n-Lie analogs of classical group theory and Lie algebra results. As an example we show that a nilpotent ideal I of class c in a n-Lie algebra A with A/I ² nilpotent of class d is nilpotent and find a bound on the class of A. We also find that some classical group theory and Lie algebra results do not hold in n-Lie algebras. In particular, non-nilpotent n-Lie algebras can admit a regular automorphism of order p, and the sum of nilpotent ideals need not be nilpotent.     

Transversal-Free Translation Nets

Translation planes of order q ² containing non-Desarguesian Baer subplanes are used to construct transversal-free translation nets with very small deficiencies. Also, a generalization of the ideas of Bruen shows that any non-Desarguesian spread in PG(3, q) produces a transversal-free net of small deficiency.