arXiv subjects
Publications and source records attributed to M. V. Lawson.
We generalize free monoids by defining $k$-monoids. These are nothing other than the one-vertex higher-rank graphs used in $C^{\ast}$-algebra theory with the cardinality requirement waived. The $1$-monoids are precisely the free monoids. We then take the next step and generalize $k$-monoids in such a way that self-similar group actions yield monoids of this type.
We take advantage of the correspondence between pseudogroups and inverse quantal frames, and of the recent description of Morita equivalence for inverse quantal frames in terms of biprincipal bisheaves, to define Morita equivalence for pseudogroups and to investigate its applications. In particular, two pseudogroups are Morita equivalent if and only if their corresponding localic étale groupoids are. We explore the clear analogies between our definition of Morita equivalence for pseudogroups and the usual notion of strong Morita equivalence for $C^{\ast}$-algebras and these lead to a number of concrete results.
We introduce what we call `generalized higher rank $k$-graphs' as a class of categories equipped with a notion of size. They extend not only the higher rank $k$-graphs, but also the Levi categories introduced by the first author as a categorical setting for graphs of groups. We prove that examples of generalized higher rank $k$-graphs can be constructed using Zappa-Szép products of groupoids and higher rank graphs.
Self-similar group actions may be encoded by a class of left cancellative monoids called left Rees monoids, a result obtained by combining pioneering work by Perrot with later work by the first author. Left Rees monoids that are also right cancellative are called Rees monoids. Irreducible Rees monoids have the striking property that they are embedded in their universal groups and these universal groups are HNN extensions by a single stable letter. In this paper, we generalize the above theory from monoids to categories. We study the structure of left Rees categories and prove that Rees categories embed into their universal groups. Furthermore, we show that from each graph of groups, we may construct a Rees category and prove that the fundamental group of the former is the universal group of the latter. In this way, Bass-Serre theory may be viewed as a special case of the general problem of embedding categories into groupoids.
Paterson showed how to construct an etale groupoid from an inverse semigroup using ideas from functional analysis. This construction was later simplified by Lenz. We show that Lenz's construction can itself be further simplified by using filters: the topological groupoid associated with an inverse semigroup is precisely a groupoid of filters. In addition, idempotent filters are closed inverse subsemigroups and so determine transitive representations by means of partial bijections. This connection between filters and representations by partial bijections is exploited to show how linear representations of inverse semigroups can be constructed from the groups occuring in the associated topological groupoid.