9 research outputs found
Quivers of monoids with basic algebras
We compute the quiver of any monoid that has a basic algebra over an
algebraically closed field of characteristic zero. More generally, we reduce
the computation of the quiver over a splitting field of a class of monoids that
we term rectangular monoids (in the semigroup theory literature the class is
known as ) to representation theoretic computations for group
algebras of maximal subgroups. Hence in good characteristic for the maximal
subgroups, this gives an essentially complete computation. Since groups are
examples of rectangular monoids, we cannot hope to do better than this.
For the subclass of -trivial monoids, we also provide a semigroup
theoretic description of the projective indecomposables and compute the Cartan
matrix.Comment: Minor corrections and improvements to exposition were made. Some
theorem statements were simplified. Also we made a language change. Several
of our results are more naturally expressed using the language of Karoubi
envelopes and irreducible morphisms. There are no substantial changes in
actual result
Complete reducibility of pseudovarieties
The notion of reducibility for a pseudovariety has been introduced as an abstract property which may be used to prove decidability results for various pseudovariety constructions. This paper is a survey of recent results
establishing this and the stronger property of complete reducibility for specific pseudovarieties.FCT through the Centro de Matemática da Universidade do Minho and Centro de Matemática
da Universidade do Port
Homogeneity and omega-categoricity of semigroups
In this thesis we study problems in the theory of semigroups which arise from model theoretic notions. Our focus will be on omega-categoricity and homogeneity of semigroups, a common feature of both of these properties being symmetricity. A structure is homogeneous if every local symmetry can be extended to a global symmetry, and as such it will have a rich automorphism group.
On the other hand, the Ryll-Nardzewski Theorem dictates that omega-categorical structures have oligomorphic automorphism groups.
Numerous authors have investigated the homogeneity and omega-categoricity of algebras including groups, rings, and of relational structures such as graphs and posets. The central aim of this thesis is to forge a new path through the model theory of semigroups.
The main body of this thesis is split into two parts. The first is an exploration into omega-categoricity of semigroups. We follow the usual semigroup theoretic method of analysing Green's relations on an omega-categorical semigroup, and prove a finiteness condition on their classes. This work motivates a generalization of characteristic subsemigroups, and subsemigroups of this form are shown to inherit omega-categoricity. We also explore methods for building omega-categorical semigroups from given omega-categorical structures.
In the second part we study the homogeneity of certain classes of semigroups, with an emphasis on completely regular semigroups. A complete description of all homogeneous bands is achieved, which shows them to be regular bands with homogeneous structure semilattices. We also obtain a partial classification of homogeneous inverse semigroups. A complete description can be given in a number of cases, including inverse semigroups with finite maximal subgroups, and periodic commutative inverse semigroups. These results extend the classification of homogeneous semilattices by Droste, Truss, and Kuske. We pose a number of open problems, that we believe will open up a rich subsequent stream of research