Purity in algebra

We investigate pure monomorphisms and pure epimorphisms in varieties of universal algebras with applications to equationally compact (= pure injective) algebras, pure projective algebras and perfect varieties. Received December 3, 2004; accepted in final form November 19, 2005. The second author was supported by the Grant Agency of the Czech Republic under the grant 201/02/0148. The hospitality of the Catholic University Louvain is gratefully acknowledged.     

Krull–Gabriel Dimension of 1-domestic String Algebras

We classify the indecomposable pure injective modules over a wide class of -domestic string algebras and calculate the Krull–Gabriel dimension of these algebras.This paper was written during the visit of the second author to the University of Manchester supported by EPSRC grant GR/R44942/01.     

Free algebras in varieties of BL-algebras generated by a chain.

In this paper we describe finitely generated free algebras in varieties of BL-algebras generated by one BL-chain which is an ordinal sum of a finite MV-chain and a generalized BL-chain. We also give some particular examples of these free algebras.     

The centers of generic division algebras with involution

The main goal of this paper is a study of the centers of the generic central simple algebras with involution. These centers are shown to be invariant fields under finite groups in a way analagous to the center of the generic division algebras. The centers of the generic central simple algebras with involution are also described as generic splitting fields (i.e. function fields of Brauer-Severi varieties) over the centers of generic division algebras. Finally, a generic central simple algebra is described for the class of central simple algebras with subfields of a certain dimension. The first author would like to thank the Department of Mathematics of The University of Texas at Austin for its hospitality and the NSF for its support under grant DMS 585-05767. The second author would like to thank the NSF for its support under grants DMS 8303356 and DMS 8601279.     

Co-Frobenius Hopf algebras and the coradical filtration

We prove that a Hopf algebra with a finite coradical filtration is co-Frobenius. We also characterize co-Frobenius Hopf algebras with coradical a Hopf subalgebra. Let H be a Hopf algebra whose coradical is a Hopf algebra. Let gr H be the associated graded coalgebra and let R be the diagram of H, c. f. [2]. Then the following are equivalent: (1) H is co-Frobenius; (2) gr H is co-Frobenius; (3) R is finite dimensional; (4) the coradical filtration of H is finite. This Theorem allows us to give systematically examples of co-Frobenius Hopf algebras, and opens the way to the classification of ample classes of such Hopf algebras. Received: 8 March 2001 / Published online: 8 November 2002 RID="*" ID="*" Parts of this work were done during visits of the second author to the University of Córdoba, Argentina, in May 2000; and of the first author to the University of Bucharest in November 2000. We thank the FOMEC and CNCSIS (Grant C12) for support to these visits. The first author also thanks ANPCyT, CONICET, Agencia Córdoba Ciencia and Secyt (UNC) for partial support. The second author was partiall y supported by Grant SM10/01 of the Research Administration of Kuwait University.     

Weak effect algebras

Weak effect algebras are based on a commutative, associative and cancellative partial addition; they are moreover endowed with a partial order which is compatible with the addition, but in general not determined by it. Every BL-algebra, i.e. the Lindenbaum algebra of a theory of Basic Logic, gives rise to a weak effect algebra; to this end, the monoidal operation is restricted to a partial cancellative operation. We examine in this paper BL-effect algebras, a subclass of the weak effect algebras which properly contains all weak effect algebras arising from BL-algebras. We describe the structure of BL-effect algebras in detail. We thus generalise the well-known structure theory of BL-algebras. Namely, we show that BL-effect algebras are subdirect products of linearly ordered ones and that linearly ordered BL-effect algebras are ordinal sums of generalised effect algebras. The latter are representable by means of linearly ordered groups. This research was partially supported by the German Science Foundation (DFG) as part of the Collaborative Research Center “Computational Intelligence” (SFB 531).     

Graded Lie algebras of maximal class. V

We describe the isomorphism classes of certain infinite-dimensional graded Lie algebras of maximal class, generated by an element of weight one and an element of weight two, over fields of even characteristic. Partially supported by MURST (Italy) via project “Graded Lie algebras and pro-p-groups of finite width”. The first author is a member of GNSAGA-INdAM. The second author is grateful to the Department of Mathematics of the University of Trento for their kind hospitality, and to MURST (Italy) for financial support.     

Algebras generated by two bounded holomorphic functions

We study the closure in the Hardy space or the disk algebra of algebras generated by two bounded functions, one of which is a finite Blaschke product. We give necessary and sufficient conditions for density or finite codimension (of the closure) of such algebras. The conditions are expressed in terms of the inner part of a certain function which is explicitly derived from each pair of generators. Our results are based on identifyingz-invariant subspaces included in the closure of the algebra. The second-named author thanks the University at Albany, Harvard University, and Brown University for their hospitality during the completion of this work.     

Global Weyl modules for the twisted loop algebra

We define global Weyl modules for twisted loop algebras and analyze their highest weight spaces, which are in fact isomorphic to Laurent polynomial rings in finitely many variables. We are able to show that the global Weyl module is a free module of finite rank over these rings. Furthermore we prove, that there exist injective maps from the global Weyl modules for twisted loop algebras into a direct sum of global Weyl modules for untwisted loop algebras. Relations between local Weyl modules for twisted and untwisted generalized current algebras are known; we provide for the first time a relation on global Weyl modules.     

Igusa–Todorov functions for Artin algebras

In this paper we study the behavior of the Igusa–Todorov functions for Artin algebras A with finite injective dimension, and Gorenstein algebras as a particular case. We show that the ?-dimension and ψ-dimension are finite in both cases. Also we prove that monomial, gentle and cluster tilted algebras have finite ?-dimension and finite ψ-dimension.     

Symmetrische Algebren mit injektivem Zentrum

The group ring of a finite group G over a field K is a symmetric algebra and hence an injective ring. If K is of characteristic zero, KG is semisimple. Thus the center of KG is injective. We proof in this paper: If K is of characteristic p>0, then the center of KG is injective, if and only if G is p-nilpotent and the p-Sylow-subgroups of G are abelian. We determine also the structure of these group rings. This will be done more generally for symmetric algebras which have an injective center and split over its Jacobson-radical.     

Monotone clones and the varieties they determine

A finite, nontrivial algebra is order-primal if its term functions are precisely the monotone functions for some order on the underlying set. We show that the prevariety generated by an order-primal algebra P is relatively congruence-distributive and that the variety generated by P is congruence-distributive if and only if it contains at most two non-ismorphic subdirectly irreducible algebras. We also prove that if the prevarieties generated by order-primal algebras P and Q are equivalent as categories, then the corresponding orders or their duals generate the same order variety. A large class of order-primal algebras is described each member of which generates a variety equivalent as a category to the variety determined by the six-element, bounded ordered set which is not a lattice. These results are proved by considering topological dualities with particular emphasis on the case where there is a monotone near-unanimity function.This research was carried out while the third author held a research fellowship at La Trobe University supported by ARGS grant B85154851. The second author was supported by a grant from the NSERC.     

Quasiequational Theories of Flat Algebras

We prove that finite flat digraph algebras and, more generally, finite compatible flat algebras satisfying a certain condition are finitely q-based (possess a finite basis for their quasiequations). We also exhibit an example of a twelve-element compatible flat algebra that is not finitely q-based. The first author was partially supported by the grant # 201/02/0594 of the Grant Agency of the Czech Republic, and by the Institutional grant MSM0021620839; the second author was partially supported by the grant No. Tn37877 of the Hungarian National Foundation for Scientific Research (OTHA); the third author was supported by the NSF grant # DMS-9971352.     

Types of polynomial completeness of expanded groups

The polynomial functions of an algebra preserve all congruence relations. In addition, if the algebra is finite, they preserve the labelling of the congruence lattice in the sense of Tame Congruence Theory. The question is for which algebras every congruence preserving function, or at least every function that preserves the labelling of the congruence lattice, is a polynomial function. In this paper, we investigate this question for finite algebras that have a group reduct. Presented by K. Kaarli. Received March 12, 2006; accepted in final form October 16, 2008. The second author is supported by Grant No. 144011 of the Ministry of Science of the Republic of Serbia, and the Scholarship “One-Month Visits to Austria for University Graduates” WUS-Austria, from the Austrian Ministry of Education, Science and Culture.     

The Blok–Ferreirim theorem for normal GBL-algebras and its application

Generalized basic logic algebras (GBL-algebras for short) have been introduced in [JT02] as a generalization of Hájek’s BL-algebras, and constitute a bridge between algebraic logic and ℓ-groups. In this paper we investigate normal GBL-algebras, that is, integral GBL-algebras in which every filter is normal. For these structures we prove an analogue of Blok and Ferreirim’s [BF00] ordinal sum decomposition theorem. This result allows us to derive many interesting consequences, such as the decidability of the universal theory of commutative GBL-algebras, the fact that n-potent GBL-algebras are commutative, and a representation theorem for finite GBL-algebras as poset sums of GMV-algebras, a result which generalizes Di Nola and Lettieri’s [DL03] representation of finite BL-algebras. Presented by J. G. Raftery. Received May 23, 2007; accepted in final form February 20, 2008.     

"Anneaux noetheriens a droite entiers sur un sous-anneau de leur centre"

Let R be a ring and G a finite group of automorphisms of R such that GL^-1? R. In the first section, we study the homological and injective dimensions of R ^G when R is simple. In the second section, we suppose that R is a k-algebra and study c,the transfer of the Nullstellensatz property between R and R^G In the last section, we apply these results to envelopping atgebras of Lie algebras and Weyl algebras.     

Elementary equivalent pairs of algebras associated with sets

The elementary equivalence of two full relation algebras, partition lattices or function monoids are shown to be equivalent to the second order equivalence of the cardinalities of the corresponding sets. This is shown to be related to elementary equivalence of permutation groups and ordinals. Infinite function monoids are shown to be ultrauniversal.Presented by Walter Taylor.The work of the second author was supported by a grant from the University of Cape Town Research Committee, and by the Topology Research Group from the University of Cape Town and the South African Council for Scientific and Industrial Research.     

The finite embeddability property for residuated lattices, pocrims and BCK-algebras

A class of algebras has the finite embeddability property (FEP) if every finite partial subalgebra of an algebra in the class can be embedded into a finite algebra in the class. We investigate the relationship of the FEP with the finite model property (FMP) and strong finite model property (SFMP).? For quasivarieties the FEP and the SFMP are equivalent, and for quasivarieties with equationally definable principal relative congruences the three notions FEP, FMP and SFMP are equivalent. The variety of intuitionistic linear algebras –which is known to have the FMP–fails to have the FEP, and hence the SFMP as well. The variety of integral intuitionistic linear algebras (also known as the variety of residuated lattices) does possess the FEP, and hence also the SFMP. Similarly contrasting statements hold for various subreduct classes. In particular, the quasivarieties of pocrims and of BCK-algebras possess the FEP. As a consequence, the universal theories of the classes of residuated lattices, pocrims and BCK-algebras are decidable. Received February 16, 2001; accepted in final form November 2, 2001. RID="h1" ID="h1"The second author was supported by a postdoctoral research fellowship of the National Research Foundation of South Africa, hosted by the University of Illinois at Chicago.