$$\mathcal{P}\mathcal{A}{\text{-isomorphisms}}$$ of inverse semigroups

We investigate recent uniqueness theorems for reduced \(C^*\)-algebras of Hausdorff etale groupoids in the context of inverse semigroups. In many cases the distinguished subalgebra is closely related to the structure of the inverse semigroup. In order to apply our results to full \(C^*\)-algebras, we also investigate amenability. More specifically, we obtain conditions that guarantee amenability of the universal groupoid for certain classes of inverse semigroups. These conditions also imply the existence of a conditional expectation onto a canonical subalgebra.

On a Class of Lattice Ordered Inverse Semigroups

SynopsisThe aim of this paper is to describe the free product of a pair G, H of groups in the category of inverse semigroups. Since any inverse semigroup generated by G and H is a homomorphic image of this semigroup, this paper can be regarded as asking how large a subcategory, of the category of inverse semigroups, is the category of groups? In this light, we show that every countable inverse semigroup is a homomorphic image of an inverse subsemigroup of the free product of two copies of the infinite cyclic group. A similar result can be obtained for arbitrary cardinalities. Hence, the category of inverse semigroups is generated, using algebraic constructions by the subcategory of groups.The main part of the paper is concerned with obtaining the structure of the free product G inv H, of two groups G, H in the category of inverse semigroups. It is shown in section 1 that G inv H is E-unitary; thus G inv H can be described in terms of its maximum group homomorphic image G gp H, the free product of G and H in the category of groups, and its semilattice of idempotents. The second section considers some properties of the semilattice of idempotents while the third applies these to obtain a representation of G inv H which is faithful except when one group is a non-trivial finite group and the other is trivial. This representation is used in section 4 to give a structure theorem for G inv H. In this section, too, the result described in the first paragraph is proved. The last section, section 5, consists of examples.

We examine a certain embedding problem for groups that have a presentation described by partial automorphisms. Semidirect products and HNN extensions have such a presentation. The embedding problem is closely related to systems of partial automorphisms, which are formalized by the concept of inverse semigroups. A necessary and sufficient condition for a group to be embedded in a certain sense is obtained by geometric methods using van Kampen diagrams.

We introduce a preorder on an inverse semigroup S associated to any normal inverse subsemigroup N, that lies between the natural partial order and Green’s $${\mathcal {J}}$$ –relation. The corresponding equivalence relation $$\simeq _N$$ is not necessarily a congruence on S, but the quotient set does inherit a natural ordered groupoid structure. We show that this construction permits the factorisation of any inverse semigroup homomorphism into a composition of a quotient map and a star-injective functor, and that this decomposition implies a classification of congruences on S. We give an application to the congruence and certain normal inverse subsemigroups associate to an inverse monoid presentation.

A semigroup is tight if each of its congruences is uniquely determined by eachof the congruence classes. Bisimple inverse semigroups are tight, and tightsemigroups are either simple or congruence-free with zero. Although congruence-freesemigroups are tight, they are not necessarily bisimple. We constructtight inverse semigroups and tight inverse monoids that are neither bisimplenor congruence-free.

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.

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

We consider semigroups such that the universal left congruence omega ^{ell } is finitely generated. Certainly a left noetherian semigroup, that is, one in which all left congruences are finitely generated, satisfies our condition. In the case of a monoid the condition that omega ^{ell } is finitely generated is equivalent to a number of pre-existing notions. In particular, a monoid satisfies the homological finiteness condition of being of type left-{text {FP}}_1 exactly when omega ^{ell } is finitely generated.Our investigations enable us to classify those semigroups such that omega ^{ell } is finitely generated that lie in certain important classes, such as strong semilattices of semigroups, inverse semigroups, Rees matrix semigroups (over semigroups) and completely regular semigroups. We consider closure properties for the class of semigroups such that omega ^{ell } is finitely generated, including under morphic image, direct product, semi-direct product, free product and 0-direct union. Our work was inspired by the stronger condition, stated for monoids in the work of White, of being pseudo-finite. Where appropriate, we specialise our investigations to pseudo-finite semigroups and monoids. In particular, we answer a question of Dales and White concerning the nature of pseudo-finite monoids.

On topological McAlister semigroups

A semigroup S is called E-inversive if for every a ∈ S ther is an x ∈ S such that (ax)2 = ax. A construction of all E-inversive subdirect products of two E-inversive semigroups is given using the concept of subhomomorphism introduced by McAlister and Reilly for inverse semigroups. As an application, E-unitary covers for an E-inversive semigroup are found, in particular for those whose maximum group homomorphic image is a given group. For this purpose, the explicit form of the least group congruence on an arbitrary E-inversive semigroup is given. The special case of full subdirect products of a semilattice and a group (that is, containing all indempotents of the direct product) is investigated and, following an idea of Petrich, a construction of all these semigroups is provided. Finally, all periodic semigroups which are subdirect products of a semilattice or a band with a group are characterized.

1·1. We shall write for the class of groups all of whose non-trivial normal subgroups have finite index. Thus, rather obviously, finite groups, simple groups, and the infinite cyclic and dihedral groups all lie in the class . Other examples of -groups are to be found in a variety of contexts. The main result of Mennicke (8) shows that, for n ≥ 3, the factor group of SL(n, Z) by its centre is a -group, where SL(n, Z) denotes the unimodular group of all n × n invertible matrices with integer coefficients and with determinant 1. In McLain(7), an example is given of an infinite, locally finite, locally soluble -group, whose only non-trivial normal subgroups are the terms of its derived series, and in (4), P. Hall discusses an infinite -group, all of whose proper quotients are finite p-groups. If is a class of groups closed under taking homomorphic images, it is easily seen that the existence of an infinite -group satisfying the maximal condition for normal subgroups implies the existence of an infinite -group in the class , so that certain questions concerning the finiteness of groups satisfying the maximal condition for normal subgroups can be interpreted as questions about -groups.

The co-maximal subgroup graph Γ(G), of a finite group G, is a special kind of undirected graph, in which the set of vertices contains non-trivial proper subgroups of G and two distinct vertices A and U are adjacent iff AU = G. In this paper, we discuss the structures of co-maximal subgroup graphs of finite cyclic groups ℤpn, ℤp1np2m, ℤp1np2p3, ℤpn1pm2p3 and ℤp1np2mpl3 for n, m, l ∈ N, where p1, p2 and p3 are distinct prime numbers. Also, we establish the super graceful labeling of these graphs.

Peter Schmid Mathematisches Institut der Universit~it, Auf der Morgenstelle 10, D-7400-Tiibingen1, Federal Republic of Germany To Helmut Wielandt, on his seventieth birthday, 19 December, 1980 Let A be a Q-module where A and Q are finite p-groups. By a theorem of Gaschtitz and Uchida A is cohomologically trivial provided the (Tare) cohomology H"(Q, A)=0 for just one integer n (cf. [-1], p. 110). This has been used in order to produce noninner p-automorphisms for p-groups. In fact, when N is a normal subgroup of a finite p-group G and Q = G/N, it often happens that the Q-mod- ule A =Z(N), the centre of N, has nontrivial cohomology. The object of this paper is to record the following result. Theorem. Let G be a regular p-group and N a nontrivial normal subgroup of G. If Q = G/N is not cyclic, H"(Q, Z(N)) =t= 0 for all n. If A is a cyclic p-group, p an odd prime, and Q a p-subgroup of Aut(A), the semidirect product G=Q. A is regular but Hn(Q,A)=O. Thus the hypothesis in the theorem cannot be omitted. ' One might ask when extensions of regular p-groups are aga!n regular. Some necessary condition may be taken from Proposition 2 below.' Note, however, that even direct products of regular p-groups need not be regular; see Wielandt's example in [-2], p. 323. 1. Cohomological Triviality and Fixed Points We begin with some basic observations concerning cohomologically trivial mod- ules for p-groups. If A is a (right) Q-module, A o denotes the submodule of fixed points under Q and [A,Q] the commutator submodule. The trace map a ~-+ a ~, x of A is written z = zQ, its image A t and its kernel A,. In dealing with

A Cayley snark is a cubic Cayley graph which is not 3-edge-colourable. In the paper we discuss the problem of the existence of Cayley snarks. This problem is closely related to the problem of the existence of non-hamiltonian Cayley graphs and to the question whether every Cayley graph admits a nowhere-zero 4-flow. So far, no Cayley snarks have been found. On the other hand, we prove that the smallest example of a Cayley snark, if it exists, comes either from a non-abelian simple group or from a group which has a single non-trivial proper normal subgroup. The subgroup must have index two and must be either non-abelian simple or the direct product of two isomorphic non-abelian simple groups.