1,785 research outputs found
Permutation of elements in double semigroups
Double semigroups have two associative operations related by
the interchange relation: . Kock \cite{Kock2007} (2007) discovered a
commutativity property in degree 16 for double semigroups: associativity and
the interchange relation combine to produce permutations of elements. We show
that such properties can be expressed in terms of cycles in directed graphs
with edges labelled by permutations. We use computer algebra to show that 9 is
the lowest degree for which commutativity occurs, and we give self-contained
proofs of the commutativity properties in degree 9.Comment: 24 pages, 11 figures, 4 tables. Final version accepted by Semigroup
Forum on 12 March 201
Two Generalizations of Homogeneity in Groups with Applications to Regular Semigroups
Let be a finite set such that and let . A group
G\leq \sym is said to be -homogeneous if for every ,
such that and , there exists such that .
(Clearly -homogeneity is -homogeneity in the usual sense.)
A group G\leq \sym is said to have the -universal transversal property
if given any set (with ) and any partition of
into blocks, there exists such that is a section for .
(That is, the orbit of each -subset of contains a section for each
-partition of .)
In this paper we classify the groups with the -universal transversal
property (with the exception of two classes of 2-homogeneous groups) and the
-homogeneous groups (for ). As a
corollary of the classification we prove that a -homogeneous group is
also -homogeneous, with two exceptions; and similarly, but with no
exceptions, groups having the -universal transversal property have the
-universal transversal property.
A corollary of all the previous results is a classification of the groups
that together with any rank transformation on generate a regular
semigroup (for ).
The paper ends with a number of challenges for experts in number theory,
group and/or semigroup theory, linear algebra and matrix theory.Comment: Includes changes suggested by the referee of the Transactions of the
AMS. We gratefully thank the referee for an outstanding report that was very
helpful. We also thank Peter M. Neumann for the enlightening conversations at
the early stages of this investigatio
Generalized Results on Monoids as Memory
We show that some results from the theory of group automata and monoid
automata still hold for more general classes of monoids and models. Extending
previous work for finite automata over commutative groups, we demonstrate a
context-free language that can not be recognized by any rational monoid
automaton over a finitely generated permutable monoid. We show that the class
of languages recognized by rational monoid automata over finitely generated
completely simple or completely 0-simple permutable monoids is a semi-linear
full trio. Furthermore, we investigate valence pushdown automata, and prove
that they are only as powerful as (finite) valence automata. We observe that
certain results proven for monoid automata can be easily lifted to the case of
context-free valence grammars.Comment: In Proceedings AFL 2017, arXiv:1708.0622
Symmetric Groups and Quotient Complexity of Boolean Operations
The quotient complexity of a regular language L is the number of left
quotients of L, which is the same as the state complexity of L. Suppose that L
and L' are binary regular languages with quotient complexities m and n, and
that the transition semigroups of the minimal deterministic automata accepting
L and L' are the symmetric groups S_m and S_n of degrees m and n, respectively.
Denote by o any binary boolean operation that is not a constant and not a
function of one argument only. For m,n >= 2 with (m,n) not in
{(2,2),(3,4),(4,3),(4,4)} we prove that the quotient complexity of LoL' is mn
if and only either (a) m is not equal to n or (b) m=n and the bases (ordered
pairs of generators) of S_m and S_n are not conjugate. For (m,n)\in
{(2,2),(3,4),(4,3),(4,4)} we give examples to show that this need not hold. In
proving these results we generalize the notion of uniform minimality to direct
products of automata. We also establish a non-trivial connection between
complexity of boolean operations and group theory
The finiteness of a group generated by a 2-letter invertible-reversible Mealy automaton is decidable
We prove that a semigroup generated by a reversible two-state Mealy automaton
is either finite or free of rank 2. This fact leads to the decidability of
finiteness for groups generated by two-state or two-letter
invertible-reversible Mealy automata and to the decidability of freeness for
semigroups generated by two-state invertible-reversible Mealy automata
Factorization theory: From commutative to noncommutative settings
We study the non-uniqueness of factorizations of non zero-divisors into atoms
(irreducibles) in noncommutative rings. To do so, we extend concepts from the
commutative theory of non-unique factorizations to a noncommutative setting.
Several notions of factorizations as well as distances between them are
introduced. In addition, arithmetical invariants characterizing the
non-uniqueness of factorizations such as the catenary degree, the
-invariant, and the tame degree, are extended from commutative to
noncommutative settings. We introduce the concept of a cancellative semigroup
being permutably factorial, and characterize this property by means of
corresponding catenary and tame degrees. Also, we give necessary and sufficient
conditions for there to be a weak transfer homomorphism from a cancellative
semigroup to its reduced abelianization. Applying the abstract machinery we
develop, we determine various catenary degrees for classical maximal orders in
central simple algebras over global fields by using a natural transfer
homomorphism to a monoid of zero-sum sequences over a ray class group. We also
determine catenary degrees and the permutable tame degree for the semigroup of
non zero-divisors of the ring of upper triangular matrices over a
commutative domain using a weak transfer homomorphism to a commutative
semigroup.Comment: 45 page
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