Application of verification techniques to inverse monoids

The word problem for inverse monoids generated by a set Γ subject to relations of the form e = f, where e and f are both idempotents in the free inverse monoid generated by Γ, is investigated. It is shown that for every fixed monoid of this form the word problem can be solved in polynomial time which solves an open problem of Margolis and Meakin. For the uniform word problem, where the presentation is part of the input, EXPTIME-completeness is shown. For the Cayley-graphs of these monoids, it is shown that the first-order theory with regular path predicates is decidable. Regular path predicates allow to state that there is a path from a node x to a node y that is labeled with a word from some regular language. As a corollary, the decidability of the generalized word problem is deduced. Finally, it is shown that the Cayley-graph of the free inverse monoid has an undecidable monadic second-order theory.

Inverse monoids: Decidability and complexity of algebraic questions

Free partially commutative inverse monoids are investigated. Analogously to free partially commutative monoids (trace monoids), free partially commutative inverse monoid are the quotients of free inverse monoids modulo a partially defined commutation relation on the generators. An O(n log(n)) algorithm on a RAM for the word problem is presented, and NP-completeness of the generalized word problem and the membership problem for rational sets is shown. Moreover, free partially commutative inverse monoids modulo a finite idempotent presentation are studied. For these monoids, the word problem is decidable if and only if the complement of the commutation relation is transitive.KeywordsWord ProblemInverse SemigroupCayley GraphDependence GraphDependence RelationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The word problem for inverse monoids presented by one idempotent relator

E-unitary inverse monoids and the Cayley graph of a group presentation

On a Class of Lattice Ordered Inverse Semigroups

On one-relator inverse monoids and one-relator groups

A new approach based on computing the Gröbner basis of polynomial ideals is developed for solving word problems and unification problems for finitely presented commutative algebras. This approach is simpler and more efficient than the approaches based on generalizations of the Knuth-Bendix completion procedure to handle associative and commutative operators. It is shown that (i) the word problem over a finitely presented commutative ring with unity is equivalent to the polynomial equivalence problem modulo a polynomial ideal over the integers, (ii) the unification problem for linear forms is decidable for finitely presented commutative rings with unity, (iii) the word problem and unification problem for finitely presented boolean polynomial rings are co-NP-complete and co-NP-hard respectively, and (iv) the set of all unifiers of two forms over a finitely presented abelian group can be computed in polynomial time. Examples and results of algorithms based on the Gröbner basis computation are also reported.Key WordsWord ProblemUnification ProblemFinitely Presented AlgebrasCommutative AlgebrasGröbner BasisPolynomial IdealsTerm RewritingKnuth-Bendix Completion Procedure

Amenability of Schreier graphs and strongly generic algorithms for the conjugacy problem

We study some aspects of Schein’s theory of cosets for closed inverse subsemigroups of inverse semigroups. We establish an index formula for chains of subsemigroups, and an analogue of M. Hall’s Theorem on the number of cosets of a fixed finite index. We then investigate the relationships between the following properties of a closed inverse submonoid of an inverse monoid: having finite index; being a recognizable subset; being a rational subset; being finitely generated (as a closed inverse submonoid). A remarkable result of Margolis and Meakin shows that these properties are equivalent for a closed inverse submonoid of a free inverse monoid.

Mobius monoids are Mobius categories in the sense of Leroux having a single object. The paper explores several properties of Mobius monoids and inverse monoids as well as the links between them. The right cancellative, left rigid Mobius monoids have a special place in our study. The paper has two goals: first, an examination of Mobius functions and Mobius inversion under suitable conditions, and second, a development of special inverse monoids and inverse submonoids which arise from Mobius monoids and Mobius categories. The prototype of this development is the polycyclic monoid, more precisely the free monoid as a Mobius monoid. This leads us to the generalization of some significant results on polycyclic monoids as Meakin–Sapir’s result involving self-conjugate inverse submonoids, or as Jones-Lawson’s results involving gauge inverse submonoids of the polycyclic monoids. Regarding Mobius functions we create links between several types of these functions.

Preface 1. Cayley's theorems 2. Groups generated by reflections 3. Groups acting on trees 4. Baumslag-Solitar groups 5. Words and Dehn's word problem 6. A finitely-generated, infinite, Torsion group 7. Regular languages and normal forms 8. The Lamplighter group 9. The geometry of infinite groups 10. Thompson's group 11. The large-scale geometry of groups Bibliography Index.

Word problems over traces which are solvable in linear time

Trace monoids are obtained from free monoids by defining a subset I of pairs of letters that are allowed to commute. Most of the work of this theory is an attempt to relate the properties of these monoids to the properties of I. Following the work initiated by Buchi we show that when I is an equivalence relation (the trace monoid is then a free product of free commutative monoids) it is possible to define a second order logic whose models are the traces viewed as dependence graphs and which characterizes exactly the sets of traces that are rational. This logic essentially utilizes a predicate based on the ordering defined by the dependence graph and a predicate related to a restricted use of the comparison of cardinality.

A prefix monoid is a finitely generated submonoid of a finitely presented group generated by the prefixes of its defining relators. Important results of Guba (1997), and of Ivanov, Margolis and Meakin (2001), show how the word problem for certain one-relator monoids, and inverse monoids, can be reduced to solving the membership problem in prefix monoids of certain one-relator groups. Motivated by this, in this paper, we study the class of prefix monoids of finitely presented groups. We obtain a complete description of this class of monoids. All monoids in this family are finitely generated, recursively presented and group-embeddable. Our results show that not every finitely generated recursively presented group-embeddable monoid is a prefix monoid, but for every such monoid, if we take a free product with a suitably chosen free monoid of finite rank, then we do obtain a prefix monoid. Conversely, we prove that every prefix monoid arises in this way. Also, we show that the groups that arise as groups of units of prefix monoids are precisely the finitely generated recursively presented groups, whereas the groups that arise as Schützenberger groups of prefix monoids are exactly the recursively enumerable subgroups of finitely presented groups. We obtain an analogous result classifying the Schützenberger groups of monoids of right units of special inverse monoids. We also give some examples of right cancellative monoids arising as monoids of right units of finitely presented special inverse monoids, and we show that not all right cancellative recursively presented monoids belong to this class.