Rogers Semilattices of Finite Partially Ordered Sets

It is proved that the principal sublattice of a Rogers semilattice of a finite partially ordered set is definable. For this goal to be met, we present a generalization of the Denisov theorem concerning extensions of embeddings of Lachlan semilattices to ideals of Rogers semilattices.     

Numbered distributive semilattices

In this article, we consider several definitions of a Lachlan semilattice; i.e., a semilattice isomorphic to a principal ideal of the semilattice of computably enumerable m-degrees. We also answer a series of questions on constructive posets and prove that each distributive semilattice with top and bottom is a Lachlan semilattice if it admits a Σ ₃ ⁰ -representation as an algebra but need not be a Lachlan semilattice if it admits a Σ ₃ ⁰ -representation as a poset. The examples are constructed of distributive lattices that are constructivizable as posets but not constructivizable as join (meet) semilattices. We also prove that every locally lattice poset (in particular, every lattice and every distributive semilattice) possessing a Δ ₂ ⁰ -representation is positive.     

A method for constructing semilattices of G-compactifications

We suggest a method for constructing G-spaces X such that the semilattice of G-compactifications of X coincides with the disjoint union of two semilattices corresponding to partitions of X into subspaces in which the maximal elements are identified. This method is applied to construct examples of G-Tychonoff spaces for which the semilattices of equivariant compactifications are of fairly simple structure and contain elements which are minimal but not least.     

Representation of Hilbert algebras and implicative semilattices

In this paper we shall give a topological representation for Hilbert algebras that extend the topological representation given by A. Diego in [4]. For implicative semilattices this representation gives a full duality. We shall also consider the representation for Boolean ring.     

Subsemilattice of a Semilattice of Class Σ2(X, 8)

We give a complete classification of all XI-subsemilattices of the semilattice of the class Σ₂(X, 8). We derive formulas for calculating the numbers of semilattices of the given class.     

Orthomodular semilattices

We present a simple equational characterization of (meet) semilattices with 0 where for each element p the interval [0,p] is an orthomodular lattice or an ortholattice possibly satisfying the compatibility condition.     

Local structure of Rogers semilattices of Σ<Subscript>n</Subscript><Superscript>0</Superscript>-computable numberings

We deal in specific features of the algebraic structure of Rogers semilattices of _n ⁰-computable numberings, for n 2. It is proved that any Lachlan semilattice is embeddable (as an ideal) in such every semilattice, and that over an arbitrary non 0-principal element of such a lattice, any Lachlan semilattice is embeddable (as an interval) in it.Supported by INTAS grant No. 00-499, by FP Universities of Russia grant UR.04.01.013, and by the Grant Center for Fundamental Research (GCFR), project PD02-1.1-475.__________Translated from Algebra i Logika, Vol. 44, No. 2, pp. 148–172, March–April, 2005.     

A method for finding new sets of axioms for classes of semigroups

We introduce a general technique for finding sets of axioms for a given class of semigroups. To illustrate the technique, we provide new sets of defining axioms for groups of exponent n, bands, and semilattices.     

Injective Hulls in POSV

We determine the injective objects and hulls in the category POS_V. This category is similar to the one of join semilattices but contains all partially ordered sets. The results of this paper have applications, for instance, in the theory of (generalized) ultrametric spaces.     

On geometric semilattices

We define geometric semilattices, a generalization of geometric lattices. The poset of independent sets of a matroid is another example. We prove several axiomatic and constructive characterizations, for example: geometric semilattices are those semilattices obtained by removing a principal filter from a geometric lattice. We also show that all geometric semilattices are shellable, unifying and extending several previous results.Partially supported by NSF grant MCS 81-03474.     

A jump inversion theorem for the semilattices of sigma-degrees

We prove an analogue of the jump inversion theorem for the semilattices of Σ-degrees of structures. As a corollary, we get a similar result for the semilattices of degrees of presentability of countable structures.     

Compactable semilattices

We characterize those semilattices that give rise to Boolean spaces on their associated spaces of ultrafilters. The class of 0-disjunctive semilattices, important in the theory of congruence-free inverse semigroups, plays a distinguished role in this theory.     

Semigroups that tree order sets on which they act

Characterizations are given of semigroupsS for which everyS-system is tree ordered and semigroupsS for which everyS-system is a free sum of tree semilattices.     

More on the Strength of Engeler’s Lemma

We introduce semilattices equipped with a partial n-ary operation ρ. A useful separation lemma for such nontrivial complete partial ρ-semilattices is proved equivalent to PIT, the prime ideal theorem. The relation of various versions of the Lemma to each other and to PIT is also explored. Supported by the Ministry of Education of the Czech Republic under the project MSM0021622409.     

Non-Complementedness and Non-Distributivity of Kleene Degrees

In this note, we study the complementedness and the distributivity of upper semilattices of Kleene degrees assuming V = L. K denotes the upper semilattice of all Kleene degrees. We prove that if V = L, then some sub upper semilattices of K are non-complemented and some are non-distributive.     

Irreducible generating sets of complete semigroups of unions B x (D) defined by semilattices of the class Σ2(X, 4)

In complete semigroups of unions B _x (D) defined by semilattices of class ??₂(X, 4), we selecte subsets of certain type on which equivalent binary relations are defined and by means of these relations, irreducible generating sets of the considered semigroups are described.     

Weakly self-injective semilattices

In this note we characterize weakly self-injective semilattices as Brouwerian semilattices which are compact in the residuated interval topology. We also characterize weakly self-injective semigroups which are semilattices of groups with trivial multiplication. Research of first author supported in part by a research grant from the Faculty Research Committee of Bowling Green State University. Research of second author supported in part by a postdoctoral fellowship in the Biomathematics Program at North Carolina State University under PHS Grant #GM-678 from NIGMS.     

Local separation in distributive semilattices

We introduce the Local Separation Property (LSP) for distributive semilattices. We show that LSP holds in many semilattices of the form Con_c A, where A is a lattice. On the other hand, we construct an abstract example of a distributive lattice without LSP. Our research is connected with the well known open problem whether every distributive algebraic lattice is isomorphic to the congruence lattice of some lattice. Received December 10, 2004; accepted in final form June 6, 2005.