Diagram Monoids and Their Congruences

Diagram monoids have over the last few years come into sharp focus, because of their fundamental role in the formation of diagram algebras, which in turn have applications in representation theory and theoretical physics, and also because of their intriguing algebraic and combinatorial properties. Until recently, investigations were largely concerned with the basic semigroup-theoretic and combinatorial properties of these monoids, for instance determination of Green's equivalences, computations of minimal generating sets and defining relations, and characterisations and counting of idempotents. PI (NR) and James East (JE) have recently started a collaboration on another important aspect of these monoids, namely their congruences. In a ground-breaking paper (joint with J.D. Mitchell and M. Torpey) they classified all the congruences on all classical diagram monoids over a finite set. In the process they started developing what looks like a promising general theory of semigroup congruences, at least for finite semigroups. Arising from this work are a number of strands, each important in its own right, and the project proposed here is designed to enable these strands to be followed up and for their full potential to be realised. We have identified four work packages -- dealing with infinite partition monoids, infinite diagram monoids, finite and infinite twisted diagram monoids, and development of a general theory of congruences -- which will be investigated in the course of four research visits by NR to JE over a period of two years.

The main impact will be on the academic areas to which the proposed project belongs: theory of semigroups, diagram algebras, algebra in general, and combinatorics. Because of strong links between semigroup theory and discrete mathematics on one side, and information technology on the other (particularly via theoretical computer science, theory of automata and languages, complexity, etc.) there will be realistic, secondary impacts on the latter. Finally, there will be a tangible impact on the UK mathematics and research overall as a consequence of a high-powered, and extremely timely research proposed here.