Trivial Semigroups Research Articles

Decomposition of pseudo-uninorms with continuous underlying functions via ordinal sum

The decomposition of all pseudo-uninorms with continuous underlying functions, defined on the unit interval, via Clifford's ordinal sum is described. It is shown that each such pseudo-uninorm can be decomposed into representable and trivial semigroups, and special semigroups defined on two points, where the corresponding semigroup operation is the projection to one of the coordinates. Linear orders, for which the ordinal sum of such semigroups yields a pseudo-uninorm, are also characterized.

Representation of non-commutative, idempotent, associative functions by pair-orders

The non-commutative, idempotent, associative functions are studied. It is well known that each commutative, idempotent (internal), associative function can be represented by a partial (linear) order. In this work it is shown that each non-commutative, idempotent (internal), associative function can be represented by a (linear) pair-order. Moreover, each internal, associative function can be expressed as an ordinal sum of trivial semigroups and semigroups, where the corresponding semigroup operation is the projection to one of the coordinates. Vice versa, each linear pair-order induces a unique internal, associative function and a condition under which each (non-linear) pair-order defined on a chain induces a unique monotone, idempotent, associative function is also introduced. Several examples of non-commutative, idempotent, associative functions and related pair-orders are shown.

Decomposition of idempotent pseudo-uninorms via ordinal sum

The decomposition of idempotent pseudo-uninorms is investigated. We show that each idempotent pseudo-uninorm on the unit interval can be decomposed into an ordinal sum of trivial semigroups and non-commutative idempotent semigroups defined on two elements, where the corresponding semigroup operation is the projection to one of the coordinates. Linear orders yielding idempotent pseudo-uninorms via ordinal sum of this type of semigroups are also investigated. The link between linear orders corresponding to an idempotent pseudo-uninorm and its dual pseudo-uninorm is shown for two types of duality.

Catalan monoids inherently nonfinitely based relative to finite [formula omitted]-trivial semigroups

We show that the 42-element monoid of all partial order preserving and extensive injections on the 4-element chain is not contained in any variety generated by a finitely based finite R-trivial semigroup. This provides unified proofs for several known facts and leads to a bunch of new results on the Finite Basis Problem for finite R- and J-trivial semigroups.

Full Transformation Semigroup of A Free Left S-Act on N-Generators

It is well known that the wreath product is the endmorphism monoid of a free S-act with n-generators. If S is a trivial semigroup then is isomorphic to . The extension for to where is an independent algebra has been investigated. In particular, we consider is to be , where is a free left S-act of n-generators. The eventual goal of this paper is to show that is an endomorphism monoid of a free left S-act of n-generators and to prove that is embedded in the wreath product .

On join irreducible $J$-trivial semigroups

A pseudovariety of semigroups is join irreducible if, whenever it is contained in the complete join of some pseudovarieties, then it is contained in one of the pseudovarieties. A finite semigroup is join irreducible if it generates a join irreducible pseudovariety. New finite $\mathscr{J}$-trivial semigroups $\mathcal{C}\_n$ ($n \geq 2$) are exhibited with the property that, while each $\mathcal{C}\_n$ is not join irreducible, the monoid $\mathcal{C}\_n^I$ is join irreducible. The monoids $\mathcal{C}\_n^I$ are the first examples of join irreducible $\mathscr{J}$-trivial semigroups that generate pseudovarieties that are not self-dual. Several sufficient conditions are also established under which a finite semigroup is not join irreducible. Based on these results, join irreducible pseudovarieties generated by a $\mathscr{J}$-trivial semigroup of order up to six are completely described. It turns out that besides known examples and those generated by $\mathcal{C}\_2^I$ and its dual monoid, there are no further examples.

A note on simplification of z-ordinal sum construction

We study the properties of z-ordinal sum construction and show that it can always be expressed in the reduced basic form. This means that it is enough to assume only trivial semigroups in the branching set and we can always remove semigroups with duplicate carriers. We also investigate the cardinality of the minimal branching set corresponding to monotone (and non-monotone) functions defined on the unit interval constructed via z-ordinal sum construction.

A note on moduli spaces of conformal classes for flat tori of higher dimension and on their conformal multiplication

Motivated by the theory of complex multiplication of abelian varieties, in this paper we study the conformality classes of flat tori in {mathbb {R}}^{n} and investigate criteria to determine whether a n-dimensional flat torus has non trivial (i.e. bigger than {mathbb {Z}}^{*}={mathbb {Z}}{setminus }{0}) semigroup of conformal endomorphisms (the analogs of isogenies for abelian varieties). We then exhibit several geometric constructions of tori with this property and study the class of conformally equivalent lattices in order to describe the moduli space of the corresponding tori.

Equidivisible pseudovarieties of semigroups

We give a complete characterization of pseudovarieties of semigroups whose finitely generated relatively free profinite semigroups are equidivisible. Besides the pseudovarieties of completely simple semigroups, they are precisely the pseudovarieties that are closed under Mal'cev product on the left by the pseudovariety of locally trivial semigroups. A further characterization which turns out to be instrumental is as the non-completely simple pseudovarieties that are closed under two-sided Karnofsky-Rhodes expansion.

The identities of the free product of two trivial semigroups

We exhibit an example of a finitely presented semigroup S with a minimum number of relations such that the identities of S have a finite basis while the monoid obtained by adjoining 1 to S admits no finite basis for its identities. Our example is the free product of two trivial semigroups.

An upper bound for the power pseudovariety PCS

It is a celebrated result in finite semigroup theory that the equality of pseudovarieties PG = BG holds, where PG is the pseudovariety of finite monoids generated by all power monoids of finite groups and BG is the pseudovariety of all block groups, that is, the pseudovariety of all finite monoids all of whose regular \({\fancyscript{D}}\)-classes have the property that the corresponding principal factors are inverse semigroups. Moreover, it is well known that BG = JⓜG, where JⓜG is the pseudovariety of finite monoids generated by the Mal’cev product of the pseudovarieties J and G of all finite \({\fancyscript{J}}\)-trivial monoids and of all finite groups, respectively. In this paper, a more general kind of finite semigroups is considered; namely, the so-called aggregates of block groups are introduced. It follows that the class AgBG of all aggregates of block groups forms a pseudovariety of finite semigroups. It is next proved that AgBG = JⓜCS, where JⓜCS is the pseudovariety of finite semigroups generated by the Mal’cev product of the pseudovarieties J and CS, whilst, this once, J stands for the pseudovariety of all finite \({\fancyscript{J}}\)-trivial semigroups and CS stands for the pseudovariety of all finite completely simple semigroups. Furthermore, it is shown that the power pseudovariety PCS, which is the pseudovariety of finite semigroups generated by all power semigroups of finite completely simple semigroups, has the property that \({{\bf PCS}\subseteq{\bf AgBG}}\). However, the question whether this inclusion is strict or not is left open.

Symmetric inverse topological semigroups of finite rank ≤ n

We establish topological properties of the symmetric inverse topological semigroup of finite transformations of the rank ≤ n. We show that the topological inverse semigroup is algebraically h -closed in the class of topological inverse semigroups. Also we prove that a topological semigroup S with countably compact square S×S does not contain the semigroup for infinite cardinal λ and show that the Bohr compactification of an infinite topological symmetric inverse semigroup of finite transformations of the rank ≤ n is the trivial semigroup.

Complete reducibility of systems of equations with respect to R

It is shown that the pseudovariety \mathsf R of all finite \mathcal R -trivial semigroups is completely reducible with respect to the canonical signature. Informally, if the variables in a finite system of equations with rational constraints may be evaluated by pseudowords so that each value belongs to the closure of the corresponding rational constraint and the system is verified in \mathsf R , then there is some such evaluation which is “regular”, that is one in which, additionally, the pseudowords only involve multiplications and \omega -powers.

Pointlike sets with respect to R and J

We present an algorithm to compute the pointlike subsets of a finite semigroup with respect to the pseudovariety R of all finite R -trivial semigroups. The algorithm is inspired by Henckell’s algorithm for computing the pointlike subsets with respect to the pseudovariety of all finite aperiodic semigroups. We also give an algorithm to compute J -pointlike sets, where J denotes the pseudovariety of all finite J -trivial semigroups. We finally show that, in contrast with the situation for R , the natural adaptation of Henckell’s algorithm to J computes pointlike sets, but not all of them.

Varieties of Structurally Trivial Semigroups III

The skeleton of the lattice of all structurally trivial semigroup varieties is known to be isomorphic to an infinitely ascending inverted pyramid (Kopamu, 2003). We digitize the skeleton by representing each variety forming the skeleton as an ordered triple of non-negative integers. This digitization of the lattice, under the pointwise ordering of non-negative integers, provides useful algorithms which could easily be programmed into a computer, and then used to compute varietal joins and meets, or even to draw skeleton lattice diagrams. An application to a certain larger subvariety lattice is also given as an example.

An automata-theoretic approach to the word problem for ω -terms over R

This paper studies the pseudovariety R of all finite R -trivial semigroups. We give a representation of pseudowords over R by infinite trees, called R -trees. Then we show that a pseudoword is an ω -term if and only if its associated tree is regular ( i.e. it can be folded into a finite graph), or equivalently, if the ω -term has a finite number of tails. We give a linear algorithm to compute a compact representation of the R -tree for ω -terms, which yields a linear solution of the word problem for ω -terms over R . We finally exhibit a basis for the ω -variety generated by R and we show that there is no finite basis. Several results can be compared to recent work of Bloom and Choffrut on long words.

Patterns on numerical semigroups

We introduce the notion of pattern for numerical semigroups, which allows us to generalize the definition of Arf numerical semigroups. In this way infinitely many other classes of numerical semigroups are defined giving a classification of the whole set of numerical semigroups. In particular, all semigroups can be arranged in an infinite non-stabilizing ascending chain whose first step consists just of the trivial semigroup and whose second step is the well-known class of Arf semigroups. We describe a procedure to compute the closure of a numerical semigroup with respect to a pattern. By using the concept of system of generators associated to a pattern, we construct recursively a directed acyclic graph with all the semigroups admitting the pattern.

Varieties and Nilpotent Extensions of Permutative Periodic Semigroups

We investigate certain semigroup varieties formed by nilpotent extensions of orthodox normal bands of commutative periodic groups. Such semigroups are shown to be both structurally periodic and structurally commutative, and are therefore structurally inverse semigroups. Such semigroups are also shown to be dense semilattices of structurally group semigroups. Making use of these structure decompositions, we prove that the subvariety lattice of any variety comprised of such semigroups is isomorphic to the direct product of the following three sublattices: its sublattice of all structurally trivial semigroup varieties, its sublattice of all semilattice varieties, and its sublattice of all group varieties. We conclude, therefore, that to completely describe this lattice, we must first describe completely the lattice of all structurally trivial semigroup varieties, since the other two are well known lattices.

Varieties of structurally trivial semigroups II

Melnik [5] determined completely the lattice of all 2-nilpotent extensions of rectangular band varieties; and Koselev [4] determined a distributive sublattice formed by certain varieties of n-nilpotent extensions of left zero bands. In [2] the author described the skeleton of the lattice of all 3-nilpotent extensions of rectangular bands. We generalize these results by proving that a certain family of semigroup varieties which includes all the varieties mentioned above, and referred to here as planar varieties, consisting of certain n-nilpotent extensions of rectangular bands forms a distributive sublattice that looks somewhat like an inverted pyramid. Our proof makes use of a countably infinite family of injective endomorphisms on the lattice of all semigroup varieties that was introduced by the author in [1]. Although we do not determine completely the lattice of all n-nilpotent extensions of rectangular band varieties, our result unifies certain previously known results and provides a framework for further research.

THE WREATH PRODUCT PRINCIPLE FOR ORDERED SEMIGROUPS1

ABSTRACT Straubing's wreath product principle provides a description of the languages recognized by the wreath product of two monoids. A similar principle for ordered semigroups is given in this paper. Applications to language theory extend standard results of the theory of varieties to positive varieties. They include a characterization of positive locally testable languages and syntactic descriptions of the operations and . Next we turn to concatenation hierarchies. It was shown by Straubing that the n-th level of the dot-depth hierarchy is the variety , where is the variety of locally trivial semigroups and is the n-th level of the Straubing-Thérien hierarchy. We prove that a similar result holds for the half levels. It follows in particular that a level or a half level of the dot-depth hierarchy is decidable if and only if the corresponding level of the Straubing-Thérien hierarchy is decidable.