30,466 research outputs found

### Relatively Congruence-free Regular Semigroups

Yu, Wang, Wu and Ye call a semigroup S Ď -congruence-free, where Ď is an equivalence relation on S, if any congruence Ď on S is either disjoint from Ď or contains Ď . A congruence-free semigroup is then just an Ď-congruence-free semigroup, where Ď is the universal relation. They determined the completely regular semigroups that are Ď -congruence-free with respect to each of the Greenâs relations. The goal of this paper is to extend their results to all regular semigroups. Such a semigroup is J âcongruence-free if and only if it is either a semilattice or has a single nontrivial J -class, J, say, and either J is a subsemigroup, in which case it is congruence-free, or otherwise its principal factor is congruence-free. Given the current knowledge of congruence-free regular semigroups, this result is probably best possible. When specialized to completely semisimple semigroups, however, a complete answer is obtained, one that specializes to that of Yu et al. A similar outcome is obtained for L and R. In the case of H, only the completely semisimple case is fully resolved, again specializing to those of Yu et al

### Varieties of Restriction Semigroups and Varieties of Categories

The variety of restriction semigroups may be most simply described as that generated from inverse semigroups (S, Âˇ, â1) by forgetting the inverse operation and retaining the two operations x+ = xxâ1 and x* = xâ1x. The subvariety B of strictrestriction semigroups is that generated by the Brandt semigroups. At the top of its lattice of subvarieties are the two intervals [B2, B2M = B] and [B0, B0M]. Here, B2and B0 are, respectively, generated by the five-element Brandt semigroup and that obtained by removing one of its nonidempotents. The other two varieties are their joins with the variety of all monoids. It is shown here that the interval [B2, B] is isomorphic to the lattice of varieties of categories, as introduced by Tilson in a seminal paper on this topic. Important concepts, such as the local and global varieties associated with monoids, are readily identified under this isomorphism. Two of Tilson\u27s major theorems have natural interpretations and application to the interval [B2, B] and, with modification, to the interval [B0, B0M] that lies below it. Further exploration may lead to applications in the reverse direction

### On Lattices of Varieties of Restriction Semigroups

The left restriction semigroups have arisen in a number of contexts, one being as the abstract characterization of semigroups of partial maps, another as the âweakly left E-ampleâ semigroups of the âYork schoolâ, and, more recently as a variety of unary semigroups defined by a set of simple identities. We initiate a study of the lattice of varieties of such semigroups and, in parallel, of their two-sided versions, the restriction semigroups. Although at the very bottom of the respective lattices the behaviour is akin to that of varieties of inverse semigroups, more interesting features are soon found in the minimal varieties that do not consist of semilattices of monoids, associated with certain âforbiddenâ semigroups. There are two such in the one-sided case, three in the two-sided case. Also of interest in the one-sided case are the varieties consisting of unions of monoids, far indeed from any analogue for inverse semigroups. In a sequel, the author will show, in the two-sided case, that some rather surprising behavior is observed at the next âlevelâ of the lattice of varieties

### On Semigroups with Lower Semimodular Lattice of Subsemigroups

The question of which semigroups have lower semimodular lattice of subsemigroups has been open since the early 1960s, when the corresponding question was answered for modularity and for upper semimodularity. We provide a characterization of such semigroups in the language of principal factors. Since it is easily seen (and has long been known) that semigroups for which Green\u27s relation J is trivial have this property, a description in such terms is natural. In the case of periodic semigroupsâa case that turns out to include all eventually regular semigroupsâthe characterization becomes quite explicit and yields interesting consequences. In the general case, it remains an open question whether there exists a simple, but not completely simple, semigroup with this property. Any such semigroup must at least be idempotent-free and D-trivial

### 'Requisite irony' and the knowledge based economy: a critical discourse analysis of the drafting of education policy in the european union

This chapter makes a case for combining the critical analysis of discourse with an embrace of âself-reflexive ironyâ (Jessop, 2002, 2004a) in the investigation of the articulations between the Knowledge Based Economy (KBE) and education policy in the European Union (EU). Irony is embraced as a topic within the study of EU governance of education policy in so far as it contributes to an analysis of the activities of supranational and national actors within complex multi-scalar political structures. In addition, the implications of self-reflexive irony are considered so as to suggest a series of clarifications for the process of analysing policy texts within a Critical Discourse Analysis (CDA) framework (Fairclough,1989,1996,1999). In essence, the chapter does two things. It interrogates the contradictory strategies and sources of conflict in the production of EU scale education policy texts and questions both the significance and the stability of the articulation of education reform with KBE discourses. At the same time, the chapter argues that the production of such texts contingently but incrementally contributes to the production of a relatively stable governance framework for EU scale education policy and that it is to the significance of this that a critical discourse analysis leads

### A note on the Howson property in inverse semigroups

An algebra has the Howson property if the intersection of any two finitely
generated subalgebras is finitely generated. A simple necessary and sufficient
condition is given for the Howson property to hold on an inverse semigroup with
finitely many idempotents. In addition, it is shown that any monogenic inverse
semigroup has the Howson property.Comment: 6 page

### A Common Framework for Restriction Semigroups and Regular *-Semigroups

Left restriction semigroups have appeared at the convergence of several flows of research, including the theories of abstract semigroups, of partial mappings, of closure operations and even in logic. For instance, they model unary semigroups of partial mappings on a set, where the unary operation takes a map to the identity map on its domain. This perspective leads naturally to dual and two-sided versions of the restriction property. From a varietal perspective, these classes of semigroupsâmore generally, the corresponding classes of Ehresmann semigroupsâderive from reducts of inverse semigroups, now taking a to a+=aaâ1 (or, dually, to aâ=aâ1a, or in the two-sided version, to both). In this paper the notion of restriction semigroup is generalized to P-restriction semigroup, derived instead from reducts of regular â-semigroups (semigroups with a regular involution). Similarly, [left, right] Ehresmann semigroups are generalized to [left, right] P-Ehresmann semigroups. The first main theorem is an abstract characterization of the posets P of projections of each type of such semigroup as âprojection algebrasâ. The second main theorem, at least in the two-sided case, is that for every P-restriction semigroup S there is a P-separating representation into a regular â-semigroup, namely the âMunnâ semigroup on its projection algebra, consisting of the isomorphisms between the algebraâs principal ideals under a modified composition. This theorem specializes to known results for restriction semigroups and for regular â-semigroups. A consequence of this representation is that projection algebras also characterize the posets of projections of regular â-semigroups. By further characterizing the sets of projections âinternallyâ, we connect our universal algebraic approach with the classical approach of the so-called âYork schoolâ. The representation theorem will be used in a sequel to show how the structure of the free members in some natural varieties of (P-)restriction semigroups may easily be deduced from the known structure of associated free inverse semigroups

### The Semigroups B\u3csub\u3e2\u3c/sub\u3e and B\u3csub\u3e0\u3c/sub\u3e are Inherently Nonfinitely Based, as Restriction Semigroups

The five-element Brandt semigroup B2 and its four-element subsemigroup B0, obtained by omitting one nonidempotent, have played key roles in the study of varieties of semigroups. Regarded in that fashion, they have long been known to be finitely based. The semigroup B2 carries the natural structure of an inverse semigroup. Regarded as such, in the signature {â
, -1}, it is also finitely based. It is perhaps surprising, then, that in the intermediate signature of restriction semigroups â essentially, forgetting the inverse operation x âŚ x-1 and retaining the induced operations x âŚ x+ = xx-1 and x âŚ x* = x-1x â it is not only nonfinitely based but inherently so (every locally finite variety that contains it is also nonfinitely based). The essence of the nonfinite behavior is actually exhibited in B0, which carries the natural structure of a restriction semigroup, inherited from B2. It is again inherently nonfinitely based, regarded in that fashion. It follows that any finite restriction semigroup on which the two unary operations do not coincide is nonfinitely based. Therefore for finite restriction semigroups, the existence of a finite basis is decidable modulo monoids .
These results are consequences of â and discovered as a result of â an analysis of varieties of strict restriction semigroups, namely those generated by Brandt semigroups and, more generally, of varieties of completely r-semisimple restriction semigroups: those semigroups in which no comparable projections are related under the generalized Green relation ďż˝. For example, explicit bases of identities are found for the varieties generated by B0 and B2

### Varieties of \u3cem\u3eP\u3c/em\u3e-Restriction Semigroups

The restriction semigroups, in both their one-sided and two-sided versions, have arisen in various fashions, meriting study for their own sake. From one historical perspective, as âweakly E-ampleâ semigroups, the definition revolves around a âdesignated setâ of commuting idempotents, better thought of as projections. This class includes the inverse semigroups in a natural fashion. In a recent paper, the author introduced P-restriction semigroups in order to broaden the notion of âprojectionâ (thereby encompassing the regular *-semigroups). That study is continued here from the varietal perspective introduced for restriction semigroups by V. Gould. The relationship between varieties of regular *-semigroups and varieties of P-restriction semigroups is studied. In particular, a tight relationship exists between varieties of orthodox *-semigroups and varieties of âorthodoxâ P-restriction semigroups, leading to concrete descriptions of the free orthodox P-restriction semigroups and related structures. Specializing further, new, elementary paths are found for descriptions of the free restriction semigroups, in both the two-sided and one-sided cases

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