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This paper is a contribution to the theory of finite semigroups and their classification in pseudovarieties, which is motivated by its connections with computer science. The question addressed is what role can play the consideration of an order compatible with the semigroup operation. In the case of unions of groups, so-called completely regular semigroups, the problem of which new pseudovarieties appear in the ordered context is solved. As applications, it is shown that the lattice of pseudovarieties of ordered completely regular semigroups is modular and that taking the intersection with the pseudovariety of bands defines a complete endomorphism of the lattice of all pseudovarieties of ordered semigroups.
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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 Malcev 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.
This paper enriches the list of properties of the congruence sequences starting from the universal relation and successively performing the operations of lower $t$ and lower $k$. Three classes of completely regular semigroups, namely semigroups for which $ker{sigma}$ is a cryptogroup, semigroups for which $ker{ u}$ is a cryptogroup and semigroups for which $kappa$ is over rectangular bands, are studied. $((omega_t)_k)_t$, $((mathcal{D}_t)_k)_t$ and $((omega_k)_t)_k$ are found to be the least congruences on $S$ such that the quotient semigroups are semigroups for which $ker{sigma}$ is a cryptogroup, $ker{ u}$ is a cryptogroup and $kappa$ is over rectangular bands, respectively. The results obtained present a response to three problems in Petrich and Reillys textbook textquotelefttextquoteleft Completely Regular Semigroupstextquoterighttextquoteright.
Motivated by the question of which completely regular semigroups have context-free word problem, we show that for certain classes of languages $mathfrak{C}$(including context-free), every completely regular semigroup that is a union of finitely many finitely generated groups with word problem in $mathfrak{C}$ also has word problem in $mathfrak{C}$. We give an example to show that not all completely regular semigroups with context-free word problem can be so constructed.
Several complete congruences on the lattice L(CR) of varieties of completely regular semi- groups have been fundamental to studies of the structure of L(CR). These are the kernel relation K , the left trace relation Tl , the right trace relation Tr and their intersections KcapTl,Kcap Tr . However, with the exception of the lattice of all band varieties which happens to coincide with the kernel class of the trivial variety, almost nothing is known about the internal structure of individual K-classes beyond the fact that they are intervals in L(CR). Here we present a number of general results that are pertinent to the study of K -classes. This includes a variation of the renowned Polak Theorem and its relationship to the complete retraction V -> V cap B , where B denotes the variety of bands. These results are then applied, here and in a sequel, to the detailed analysis of certain families of K -classes. The paper concludes with results hinting at the complexity of K -classes in general, such as that the classes of relation K/Kl may have the cardinality of the continuum.
The kernel relation $K$ on the lattice $mathcal{L}(mathcal{CR})$ of varieties of completely regular semigroups has been a central component in many investigations into the structure of $mathcal{L}(mathcal{CR})$. However, apart from the $K$-class of the trivial variety, which is just the lattice of varieties of bands, the detailed structure of kernel classes has remained a mystery until recently. Kadourek [RK2] has shown that for two large classes of subvarieties of $mathcal{CR}$ their kernel classes are singletons. Elsewhere (see [RK1], [RK2], [RK3]) we have provided a detailed analysis of the kernel classes of varieties of abelian groups. Here we study more general kernel classes. We begin with a careful development of the concept of duality in the lattice of varieties of completely regular semigroups and then show that the kernel classes of many varieties, including many self-dual varieties, of completely regular semigroups contain multiple copies of the lattice of varieties of bands as sublattices.
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