arXiv subjects

J. East

Publications and source records attributed to J. East.

3 recordsLinked to original sources

arXiv2023

Heights of one- and two-sided congruence lattices of semigroups

The height of a poset $P$ is the supremum of the cardinalities of chains in $P$. The exact formula for the height of the subgroup lattice of the symmetric group $\mathcal{S}_n$ is known, as is an accurate asymptotic formula for the height of the subsemigroup lattice of the full transformation monoid $\mathcal{T}_n$. Motivated by the related question of determining the heights of the lattices of left- and right congruences of $\mathcal{T}_n$, we develop a general method for computing the heights of lattices of both one- and two-sided congruences for semigroups. We apply this theory to obtain exact height formulae for several monoids of transformations, matrices and partitions, including: the full transformation monoid $\mathcal{T}_n$, the partial transformation monoid $\mathcal{PT}_n$, the symmetric inverse monoid $\mathcal{I}_n$, the monoid of order-preserving transformations $\mathcal{O}_n$, the full matrix monoid $\mathcal{M}(n,q)$, the partition monoid $\mathcal{P}_n$, the Brauer monoid $\mathcal{B}_n$ and the Temperley-Lieb monoid $\mathcal{TL}_n$.

math.GR↗

arXiv2018

Computing finite semigroups

Using a variant of Schreier's Theorem, and the theory of Green's relations, we show how to reduce the computation of an arbitrary subsemigroup of a finite regular semigroup to that of certain associated subgroups. Examples of semigroups to which these results apply include many important classes: transformation semigroups, partial permutation semigroups and inverse semigroups, partition monoids, matrix semigroups, and subsemigroups of finite regular Rees matrix and $0$-matrix semigroups over groups. For any subsemigroup of such a semigroup, it is possible to, among other things, efficiently compute its size and Green's relations, test membership, factorize elements over the generators, find the semigroup generated by the given subsemigroup and any collection of additional elements, calculate the partial order of the $\mathscr{D}$-classes, test regularity, and determine the idempotents. This is achieved by representing the given subsemigroup without exhaustively enumerating its elements. It is also possible to compute the Green's classes of an element of such a subsemigroup without determining the global structure of the semigroup.

math.RA↗

arXiv2013

Maximal subsemigroups of the semigroup of all mappings on an infinite set

In this paper we classify the maximal subsemigroups of the \emph{full transformation semigroup} $Ω^Ω$, which consists of all mappings on the infinite set $Ω$, containing certain subgroups of the symmetric group $\sym(Ω)$ on $Ω$. In 1965 Gavrilov showed that there are five maximal subsemigroups of $Ω^Ω$ containing $\sym(Ω)$ when $Ω$ is countable and in 2005 Pinsker extended Gavrilov's result to sets of arbitrary cardinality. We classify the maximal subsemigroups of $Ω^Ω$ on a set $Ω$ of arbitrary infinite cardinality containing one of the following subgroups of $\sym(Ω)$: the pointwise stabiliser of a non-empty finite subset of $Ω$, the stabiliser of an ultrafilter on $Ω$, or the stabiliser of a partition of $Ω$ into finitely many subsets of equal cardinality. If $G$ is any of these subgroups, then we deduce a characterisation of the mappings $f,g\in Ω^Ω$ such that the semigroup generated by $G\cup \{f,g\}$ equals $Ω^Ω$.

math.RA↗