### U.W. Bangor - School of Informatics - Mathematics Preprints 1995

# Semigroup and Automata Theory

#### 95.07 : LAWSON, M.V.

### Constructing Inverse Semigroups from Category Actions

#### Abstract:

The theory in this paper was motivated by an example of an inverse semigroup important in Girard's*Geometry of Interaction*programme for linear logic. At one level, the theory is a refinement of the Wagner-Preston representation theorem: we show that every inverse semigroup is isomorphis to an inverse semigroup of all partial symmetries (of a specific type) of some structure. At another level, the theory unifies and completes two classical theories: the theory of bisimple inverse monoids created by Clifford and subsequently generalised to all inverse monoids by Leech; and the theory on 0-bisimple inverse semigroups due to Reilly and McAlister. Leech showed that inverse monoids could be described by means of a class of right cancellative categories, whereas Reilly and McAlister showed that 0-bisimple inverse semigroups could be described by means of generalised RP-systems. In this paper, we prove that every inverse semigroup can be constructed from a category acting on a set satisfying what we term the `orbit condition'.

#### Published in:

*Journal of Pure and Applied Algebra*137 (1999) 57-101.

#### 95.13 : HINES, P.M. & LAWSON, M.V.

### An application of polycyclic monoids to rings

#### Abstract:

An important technique in $K$-theory of certain $C^*$-algebras is that of "halving projections". The existence of such projections in a $C^*$-algebra $A$ leads to an isomorphism between $A$ and the ring of all $n \times n$ matrices over $A$.This technique is also employed by Girard in his work on linear logic.

In this note, we show that such a process is equivalent to the existence of a 'strong embedding' of the polycyclic monoid on two generators.

We also provide a semigroup theoretic analogue of this result.

We shall assume that the reader is familiar with the elementary theory of inverse semigroups.

#### Published in:

*Semigroup Forum*56 (1998) 146-149.

#### 95.14 : J.B. Hickey & M.V.Lawson

### Unit regular monoids

#### Abstract:

We derive necessary and sufficient conditions for a unit regular monoid to have a uniquely unit regular, idempotent separating cover.#### Published in:

*Proceedings of the Royal Society of Edinburgh*127A (1997) 127-144.