NBSAN – Past meetings

This is an archive of all previous NBSAN activities.

39

The thirty-ninth NBSAN meeting was held in Durham on Monday 15th December 2025, organised by Max Gadouleau. The speakers were:

Luna Elliott - Thompson Monoids ( )
Abstract. Thompson's group $V$ was the first known example of a finitely generated infinite simple group. It is also a natural infinite analogue of the finite symmetric groups, the topological full group of the one-sided shift, the automorphism group of the Jónsson-Tarski algebra with 2 unary operations and is in some sense the symmetry group of associativity and commutativity. As a result, this group has received a lot of attention over the past 60 years of research. In this talk I discuss the monoids which play the same four roles as $V$ except from the perspectives of partial transformations, transformations, and partial bijections (where $V$ itself corresponds to bijections). In particular, I discuss their generation, presentation, and simplicity. Joint work with Reinis Cirpons, Alex Levine, and James Mitchell.
Daniel Heath - Pretzel monoids ( | slides )
Abstract. Left adequate monoids generalise the well-studied class of inverse monoids, with a graphical description of free structures given by Kambites. It is natural then to consider graphical interpretations of presentations, akin to work by Munn and Stephen for inverse monoids. In general, we believe this to be very hard; left adequate monoids do not seem to play well with presentations (they do not form a variety, for example). Here, we take tentative steps towards understanding specific presentations, namely ones resembling the famed Margolis-Meakin expansions. We introduce and study an interesting class of monoids, defined by natural combination operations on birooted, edge-labelled directed graphs, and show that they are examples of left adequate monoids with such presentations. We term the resulting monoids pretzel monoids, due to a resemblance between the diagrams we use to represent their elements and certain baked goods... We construct each monoid from a right cancellative monoid $C$ and, with this viewpoint, one may understand pretzel monoids as a left adequate analogue of the $E$-unitary Margolis-Meakin expansions. This is joint work with Mark Kambites and Nóra Szakács.
Sarah Rees - Studying monoids that model concurrency ( | slides )
Abstract. I'll discuss joint work of mine with with Ascencio-Martin, Britnell, Duncan, Francoeur and Koutny to set up and study algebraic models of concurrent computation.

Trace monoids were introduced by Mazurkiewicz as algebraic models of Petri nets, where some pairs of actions can be applied in either of two orders and have the same effect. Ab$ More recently Koutny et al. introduced the concept of a step trace monoid, which allows the additional possibility that a pair of actions may have the same effect performed sim$

I shall introduce these monoids, discuss some problems we'd like to be able to solve, and the methods with which we are trying to solve them. In particular I'll discuss normal forms for traces, comtraces and step traces, and generalisations of Stallings folding techniques for finitely presented groups and monoids.
Yayi Zhu - Relational depth of semigroups with chain-like J-classes ( | slides )
Abstract. Stemming from research on presentations for classical transformation semigroups $T_n$, $I_n$, and $PT_n$, there have been interesting results on presentations for their subsemigroups. Examples include the smallest presentations for the subsemigroups that contain $S_n$, and presentations for proper ideals such as $I_n \setminus S_n$, and $T_n \setminus S_n$. The $\mathcal{J}$-classes of these semigroups form a chain, and the relations in these presentations lie in some specific $\mathcal{J}$-classes. We are interested in the minimal $\mathcal{J}$-class, J, whose elements are involved in the presentations, and we use 'depth' to describe the position of J within the chain. In this talk, I will present results on the relational depth of some semigroups with chain-like $\mathcal{J}$-classes.

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The thirty-eighth NBSAN meeting was held in St Andrews on Thursday 12th and Friday 23th June 2025, organised by Jung Won Cho. Full details are on the meeting webpage. The speakers were:

Gemma Crowe - Twisted right-angled Artin groups
Levent Dasar - Finitely right equated semigroups
James East - Transformation representations of diagram monoids ( slides )
Joseph Edwards - Finite presentations for quotients of plactic-like monoids
Victoria Gould - Finitary conditions for graph products ( slides )
Yann Péresse - Topologies on the symmetric inverse monoid ( slides )
Catherine Reilly - Membership problems for two-generator one-relator groups and inverse monoids
Nóra Szakács - Inverse semigroups with bounded group distortion

37

The thirty-seventh NBSAN meeting took place in Manchester on Friday 11th April, organised by Josiah Aakre, Thomas Aird and Daniel Heath. The speakers were:

Peter Higgins - Two quite different semigroup problems ( slides )
Marianne Johnson - (Non-)coherency for monoids
Alex Levine - Equations in Thompson's Group F
Emanuele Rodaro - On context-free inverse graphs and their associated groups ( slides )

36

The thirty-sixth NBSAN meeting was held in Manchester on Thursday 20th and Friday 21st June 2024, as a satellite to the 75th British Mathematical Colloquium (which also featured a Semigroups Workshop). The meeting and workshop were organised by Marianne Johnson, Mark Kambites, Dmitry Kudryavtsev, Alex Levine and Nóra Szakács. Financial support was kindly provided by the London Mathematical Society, Heilbronn Institute for Mathematical Research, the Engineering and Physical Sciences Research Council and the Department of Mathematics at the University of Manchester. The NBSAN speakers were:

Thomas Aird - The meet-stalactic and meet-taiga monoid ( | slides )
Abstract. Each "plactic-like" monoids is defined by some algorithm which takes a word and outputs a combinatorial object. The stalactic monoid and taiga monoid are two such plactic-like monoids, corresponding the stalactic tableaus and binary search trees with multiplicity respectively. In this talk, I introduce two new plactic-like monoids, the meet-stalactic monoid and the meet-taiga monoid. These monoids are defined by running their respective algorithm in both directions on a word producing two combinatorial objects. After introducing these monoids, I will present a number of results about these monoids, including a Robinson-Schensted-like Theorem. This is joint work with Duarte Ribeiro.
Matthew Brookes - Congruences on direct products of simple semigroups ( | slides )
Abstract. Direct products and congruences are basic building blocks for semigroup theory, so it is natural to ask what are the congruences for a direct product. In fact, congruences themselves are subsemigroups of direct products. One construction for subsemigroups of direct products is fibre products, so one family of congruences on a direct product may be obtained as fibre products of congruences on the factors. For simple monoids this approach describes all the congruences. As a consequence it is possible classify when every congruence on a direct product decomposes as a product of congruences on the factors. One natural question asks what is the maximum length of a chain of congruences? Generalising the fibre congruence construction to describe congruences on simple actions allows us to answer this question for direct products, in terms of the maximum lengths of chains of congruences on the factors.
Tara Macalister Brough - Preserving self-similarity in free products of semigroups ( )
Abstract. I will give an update on an ongoing project I first talked about at the 12th NBSAN meeting in 2012. I have been trying to determine the extent to which the class of automaton semigroups is closed under various standard semigroup constructions, with a special focus on the free product. In recent work with Jan Philipp Wächter and Janette Welker, we not only substantially improved on my previous free product results with Alan Cain, but also observed that our constructions all work for self-similar semigroups. I will give a brief introduction to self-similarity, describe the evolution of my understanding of free products of automaton and self-similar semigroups, with a sketch of the proof of our latest result, and discuss where to go from here.
Igor Dolinka - Prefix monoids of groups and right units of special inverse monoids ( | slides )
Abstract. Motivated by the arguably most elusive open problem in combinatorial algebra - the question of decidability of the word problem for one-relator monoids, standing unresolved for almost a century - the past few decades have seen a development of a rich theory showcasing intricate and deep connections between decision problems for finitely presented monoids, inverse monoids, and groups. Among those problems we may single out the submonoid membership problem, more generally, the rational subset membership problem, or, more specifically, membership problems for some particular submonoids such as the prefix monoid (of a finitely presented group) or the monoid of right units (of a finitely presented special inverse monoid). This talk will present recent results in this vein obtained in collaboration with Robert Gray (University of East Anglia), and is, in a broad sense, related to Gray's Morning Talk at the BMC. We obtained a full characterisation of prefix monoids of finitely presented groups, as well as some further information on these monoids, such as the groups of units and Schützenberger groups. Much less is known about the monoids right units of finitely presented special inverse monoids, although some partial information is available. In particular, the universality result for the Schützenberger groups does still hold true. I will present selected results of ongoing research in this direction, as well a number of open questions.
Luna Elliott - E-disjunctive inverse semigroups ( )
Abstract. A congruence on an inverse semigroup is called idempotent-pure if it never relates an idempotent to a non-idempotent. These congruences preserve much more of the structure of the semigroup than most congruences do. The well-known class of E-unitary inverse semigroups is rich in structure because they are precisely the inverse semigroups with a group idempotent-pure quotient.

Dually to this, it is thus natural when studying more general inverse semigroups to focus on the class of inverse semigroups which have "already been quotiented by all of their idempotent-pure congruences". This is precisely the class of E-disjunctive inverse semigroups. I will introduce E-disjunctive inverse semigroups and speak about the above structure preservation in more detail. This exploration is based on some recent work done with my collaborators Alex Levine and James Mitchell.
Herman Goulet-Ouellet - Profinite bridges between semigroup theory and symbolic dynamics ( )
Abstract. My talk will explore the relationship between free profinite semigroups and symbolic dynamics, a line of research which goes back to the work of Almeida in the early 2000s. I will start by presenting Almeida's fundamental theorem, which gives a bijection between minimal shift spaces (a central object of symbolic dynamics) and maximal regular J-classes of free profinite semigroups. I will then survey a number of interesting features of this bijection from the point of view of semigroup theory, and discuss interesting applications from the point of view of symbolic dynamics.
Ganna Kudryavtseva - Duality theory for Boolean right restriction semigroups ( | slides )
Abstract. We generalize the duality between Boolean right restriction monoids and ample source-etale categories by Cockett and Garner to the non-unital and locally compact setting. Our approach upgrades the widely known construction of the tight groupoid of an inverse semigroup as the groupoid of germs. Elements of a supported Boolean right restriction semigroup are represented by right compact slices of their attached right ample categories. In these categories, the domain map is a local homeomorphism, but the range map is not even open in general, so that ranges of elements of the category are not 'seen' in the attached right restriction semigroup. In the special case where the range map of the right ample category is open, the right restriction semigroup has the additional structure of a left Ehresmann semigroup. Specializing further to the case where the range map of the category is a local homeomorphism, the category has the additional property that every compact right slice is a finite join of compact two-sided slices. On the algebraic side, this brings Boolean right restriction Ehresmann semigroups with the extra property that every element is a finite join of deterministic elements. These semigroups are a natural generalization of Garner's groupoidal right restriction monoids.
António Malheiro - Quasi-crystals and algebraic structures: linking cyrstal bases to semigroups and beyond ( | slides )
Abstract. This talk delves into the intricate world of quasi-crystals, an extension of the well-established theory of crystal bases in representation theory. Originating from the groundbreaking work of Drinfeld, Jimbo, and Kashiwara in the 1980s, quantum groups and crystal bases have played a pivotal role in theoretical physics and various mathematical domains. Kashiwara's development of crystal bases and crystal graphs provided a framework for studying representations of quantum groups, revealing intriguing connections with Young tableaux and the plactic monoid.

The plactic monoid, central to the theory of symmetric polynomials and the Littlewood-Richardson rule, bridges the gap between crystal structures and Schur polynomials. In parallel, the hypoplactic monoid enters the scene in the realm of quasi-symmetric functions, offering an analogue to the classical plactic monoid with applications in quasi-ribbon tableaux.

Building on the quasi-crystal structure introduced with Alan Cain, this talk explores the extension of crystal concepts to quasi-crystals. We present a set of local axioms for quasi-crystal graphs of simply-laced root systems, drawing parallels with Stembridge's work on crystals. The characterization of quasi-crystal graphs arising from the quasi-crystal of type An answers an open question, providing insight into the local structural properties that define these structures.
Itamar Stein - The algebra of the monoid of order-preserving functions and other reduced E-Fountain semigroups ( | slides )
Abstract. With the monoid of all order-preserving functions on an $n$-set, we associate a category with partial composition - a category in which the composition of two morphisms might be undefined even if the range of the first equals the domain of the second. We will show that the algebra of the monoid is isomorphic to the algebra of the category over any commutative unital ring. Similar results have been useful in the study of algebras of many other finite semigroups: inverse semigroups, the monoid of partial functions, the Catalan monoid etc.

More generally, with every reduced E-Fountain semigroup which satisfies the generalized right ample condition we will show how to associate a category with partial composition. Under some assumptions, we will prove an isomorphism of algebras between the semigroup algebra and the category algebra.

This is a simultaneous generalization of a former result on reduced E-Fountain semigroups which satisfy the congruence condition, a result of Junying Guo and Xiaojiang Guo on strict right ample semigroups and a result of Benjamin Steinberg on idempotent semigroups with central idempotents.

If time allows, we will discuss additional examples.
Tim Stokes - Can constellations shed light on semigroups? ( | slides )
Abstract. A constellation is a partial algebra that is a one-sided generalised category, with each element $s$ having a domain $D(s)$ but no range in general. They model composition of functions (or morphisms) $f\circ g$ where the image of $f$ is contained (as a substructure) in the domain of $g$, and codomains play no role.

Constellations were developed by Gould and Hollings as a tool in semigroup theory for finding a category-like counterpart of left restriction semigroups. Those constellations corresponding to left restriction semigroups are called inductive, by analogy with the ordered groupoid/inverse semigroup case.

General constellations have proved of interest for their own sake. A process of right canonical extension of a constellation (which puts back in the missing codomains!) gives a category, leading to novel ways to think about the familiar concrete categories of mathematics.

We shall explain how one can use a left-sided version of the canonical extension idea to obtain constellations from monoids. If the constellation is inductive, one obtains a left restriction monoid. This leads to new ways to build some familiar left restriction monoids from their subsemigroups of elements of domain $1$. The left restriction monoid of partial functions on a set $X$ can be obtained in this way from the transformation monoid on $X$. Other examples arise from the theories of binary relations and partitions.
Jan Philipp Wächter - Decision problems for automaton semigroups and groups ( | slides )
Abstract. We will discuss the decidability and complexity of the most important decision problems for semigroups and groups generated by finite-state, letter-to-letter transducers (which are usually simply referred to as "automata" in this context). In such an automaton, every state induces a function mapping input to output words. The closure of these functions under composition is the generated semigroup. If the automaton has additional properties (such as being complete and/or invertible), we obtain groups and inverse semigroups. The study of these objects is motivated by some groups with exotic properties that can be found in this class (where Grigorchuk's group is usually considered to be the most prominent one). In the talk, we will recall the basic definitions and mainly give an overview of the most important results with regard to decision problems and also discuss some recent developments.
Magdalena Wiertel - Hecke-Kiselman monoids and algebras ( | slides )
Abstract. To every finite oriented graph $\Theta$ with $n$ vertices one can associate a finitely presented monoid $HK_{\Theta}$, called the Hecke--Kiselman monoid. It is a monoid generated by $n$ idempotents with relations of the form $xy=yx$ or $xyx=yxy=xy$, depending on the edges between vertices $x$ and $y$ in $\Theta$. By the Hecke-Kiselman algebra we mean the monoid algebra $K[HK_{\Theta}]$ of the monoid $HK_{\Theta}$ over a field $K$. We investigate the combinatorics and structure of the Hecke-Kiselman monoids and algebras. It turns out that the case of the monoid $C_n$ associated to an oriented cycle of length $n\geqslant 3$ plays a crucial role. The unexpected chain of ideals inside $C_n$ is constructed. This result is then applied to show that the monoid $C_n$ satisfies a semigroup identity and the associated algebra is a semiprime Noetherian algebra. As a consequence, we characterise all Hecke-Kiselman monoids that satisfy a semigroup identity. Moreover, the Jacobson radical of all Hecke-Kiselman algebras satisfying a polynomial identity is described.

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The thirty-fifth NBSAN meeting was held in York on Friday 17th November 2023, organised by Craig Miller. The speakers were:

Peter Faul - Fibrational approaches to Clifford Semigroups ( )
Abstract. Since it is possible to represent Clifford semigroups as functors $\mathrm{L} \to \mathrm{Grp}$, it is natural to ask if semigroup homomorphisms between them can be conceptualised in terms of maps between their associated functors. Phrased more precisely, is there a category whose objects are the functors so described and which is equivalent to the category of Clifford semigroups? While this problem has been solved, we argue in this talk that there is a more principled categorical approach that leads directly to the solution, involving the notion of a fibration. The advantage of this approach is that it brings along for the ride a number of interesting structures. For instance, this fibration may be exploited to gain a greater understanding of the morphisms between Clifford semigroups. In particular, each orthogonal factorisation system on the codomain lifts to an orthogonal factorisation system on the domain category. We are able to conclude that every homomorphism between Clifford semigroups can be factored as a `surjective cartesian map' followed by an `idempotent-fixing injection'. This restricts easily to the subcategory of commutative inverse semigroups.
Robert Gray - Symmetries of Schützenberger graphs and subgroups of special inverse monoids ( )
Abstract. In the 1960's Higman was able to characterize the finitely generated subgroups of finitely presented groups. His result, which is called the Higman Embedding Theorem, is a key result in combinatorial group theory which makes precise the connection between group presentations and logic. In this talk I will present some results of a similar flavour, proved in recent joint work with Mark Kambites (Manchester), in which we characterise the maximal subgroups of inverse monoids defined by presentations where all the defining relators are of the form $w=1$. I will explain the motivation for studying this class of inverse monoids, and also indicate how we established these results by developing new geometric approaches to maximal subgroups, exploiting the fact that they are isomorphic to automorphism groups of Schutzenberger graphs. I will also say something about some related recent joint work with Igor Dolinka (Novi Sad) on, so-called, prefix monoids and their connection with the class of recursively presented group-embeddable monoids.
Jung Won Cho - Subsemigroups of the free monogenic inverse semigroup ( )
Abstract. Abstract: In 1975, Boris Schein proved that free inverse semigroups are not finitely presentable as semigroups. In particular, he showed that the free monogenic inverse semigroup is not finitely presentable using two particular partial transformations. Motivated by this result, I will give a proof that a subsemigroup (not necessarily inverse) of the free monogenic inverse semigroup is finitely presented if and only if it contains only finitely many idempotents. This is joint work with Nik Ruškuc.
Robert Kropholler - Multiple context-free groups are closed under free products ( )
Abstract. A group is multiple context-free, if its word problem is a multiple context-free language (MCFL). MCFLs sit between the class of context-free languages and context sensitive languages. By a theorem of Muller and Schupp a group is context-free if and only if it is virtually free. In 2015 Salvati showed that $\mathbb{Z}^2$ is a multiple context-free group. From there, Ho has shown that all virtually abelian groups are in this class. It would be of interest to have a classification similar to that of Muller and Schupp. I will discuss one step of this, namely showing that this class is closed under free products. I will mostly focus on proving this result for context-free languages using pushdown automata and discuss the modifications and automata needed for the multiple context-free case. This is joint work with Davide Spriano.
Catarina Monteiro - Formations and i-Fitting classes of inverse semigroups, congruences and languages ( )
Abstract. A class of finite groups $\mathcal{F}$ closed under quotients and finitary subdirect products is said to be a formation of finite groups. A class of finite groups $\mathcal{C}$ closed under subnormal subgroups and for groups generated by two normal subgroups which are in $\mathcal{C}$ is said to be a Fitting class of finite groups. A natural question that arises is whether there are corresponding concepts for congruences and languages. This question has been addressed for formations of groups, of monoids and of many-sorted algebras, and bijections between formations of semigroups, of congruences on the associated free object and of languages have been obtained. In this talk, based on recent join work with G. Gomes, I will show that there exist bijections between formations of inverse semigroups, formations of congruences on inverse semigroups, and formations of congruences on free inverse semigroups. Further, I will prove the existence of bijections between i-formations of inverse semigroups, of (idempotent separating) congruences and languages (contained in the centraliser of the semigroup). This study appears in "Formations and i-Fitting classes of inverse semigroups, congruences and languages", and it was motivated by those developed in "Formations of inverse semigroups" and "Formations of Orthodox Semigroups", where formations associated to idempotent-separating congruences play an essential role. Based on these results, and with Fitting classes being the dual concept of formations, it is natural to look for similar results to those obtained for formations. I will prove the existence of bijections between i-Fitting classes of Clifford semigroups, of (idempotent separating) congruences, and of languages (contained in the centralizer).
Nasir Sohail - Amalgamating inverse semigroups over ample semigroups ( )
Abstract. It was shown by Howie that a semigroup amalgam $ (S;T_1,T_2) $ does not embed if $ T_1 $ and $ T_2$ are both groups but $ S $ is not a group. Generalizing this result, Rahkema and Sohail showed that $(S;T_1,T_2)$ is non-embeddable if $T_1$ and $T_2$ are completely regular (respectively, Clifford) semigroups but $S$ is not completely regular (respectively, Clifford). In this talk we shall consider the amalgams $ (S;T_1,T_2) $ such that $T_1$ and $ T_2 $ are inverse semigroups while S is non-inverse but ample.

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The thirty-fourth NBSAN meeting was held in Norwich on Thursday 15th and Friday 16th June 2023, organised by Robert D. Gray and Islam Foniqi, and with a focus on decision problems in group and monoid theory. Full details are on the dedicated meeting webpage. The speakers were:

María Cumplido Cabello - Solving the word problem in Artin groups without braid relations
Laure Daviaud - Some applications of semigroup theory in formal verification
Lorna Gregory - Representation type and decidability of theories of modules of finite-dimensional algebras
Martin Hampenberg Christensen - Submonoids of the symmetric inverse monoid
Alan Logan - Post's correspondence problem in group theory
Carl-Fredrik Nyberg Brodda - One relator, many problems
Igor Potapov - Matrix semigroups, equations and linear maps
Daniel Turaev - Deciding the first order theory of the plactic monoid

33

The thirty-third NBSAN meeting was held in Manchester on Friday 14th April 2023, organised by Mark Kambites and Nóra Szakács. The speakers were:

Ruy Exel - The tight $C^*$-algebra of an inverse semigroup ( )
Abstract. I'll describe the motivation for and the construction of the tight $ C^* $-algebra of an inverse semigroup as well as a few examples where the construction leads to $ C^* $-algebras of interest.
Islam Foniqi - Decision problems for one-relator monoids and groups ( | slides )
Abstract. The word problem for one-relator monoids is still open. This motivates the study of other decision problems for monoids, inverse monoids and groups. We will provide some recent results on submonoid, and rational subset membership problems in algebraic structures, and a discussion of their implications and connections.
Victoria Gould - Pseudo-finite semigroups and diameter ( | slides )
Abstract. A semigroup $ S $ is said to be (right) pseudo-finite if the universal right congruence $ \omega_S:= S\times S $ can be generated by a finite set $ U\subseteq S\times S $, and there is a bound on the length of derivations for an arbitrary pair $ (s,t)\in S\times S $ as a consequence of those in $ U $. The diameter of a pseudo-finite semigroup is the smallest such bound taken over all finite generating sets. The notion of being pseudo-finite was introduced by White in the language of ancestry, motivated by a conjecture of Dales and Żelazko for Banach algebras. The property also arises from a number of other sources.

Pseudo-finiteness is a very unpredictable condition. In the presence of cancellativity, e.g. for a group, it forces finiteness and, consequently, diameter 1. Any monoid with zero is easily seen to be pseudo-finite with diameter at most 2. Rather trivially, both kinds of examples have minimal ideals. Further, any full transformation monoid $ \mathcal{T}_X $ (for any cardinality of $ X $ ) is pseudo-finite with diameter 1, and has a minimal ideal $ I $ of constant maps on which $ \mathcal{T}_X $ acts with a high degree of transitivity.

The talk introduces the curious world of pseudo-finite semigroups. It has two focusses: exploring the existence and nature of a minimal ideal in a pseudo-finite semigroup, and the size of the diameter. Actions, presentations, Rees matrix constructions and some good old-fashioned semigroup tools will all play a part. This research sits in the wider framework of a study of finitary conditions for semigroups.

This is joint work with James East, Craig Miller, Nik Ruškuc and Tom Quinn-Gregson.
Mark Kambites - Subgroups of special inverse monoids ( )
Abstract. I will talk about recent joint work with Robert Gray on the subgroup structure of finitely presented special inverse monoids.
Xin Li - $C^*$-algebras and dynamical systems arising from semigroups ( )
Abstract. The first part of the talk will be an overview, with a focus on examples, of semigroup $C^*$-algebras, which are $C^*$-algebras generated by left regular representations of semigroups. The second part of the talk will discuss underlying dynamical systems which are interesting in their own right.

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The thirty-second NBSAN meeting was held in York on Friday 24th June 2022, organised by Victoria Gould and in conjunction with a workshop on Topics in Geometric Semigroup Theory the same week. The speakers were:

Fabienne Chouraqui - About Garside monoids and M-braces ( )
Abstract. We define an algebraic structure similar to that of a semiring, but without some of the requirements. As it is somehow also similar to the structure of left brace, we call it an $ \mathscr{M} $-brace. We present a connection between Garside monoids and more generally lcm-monoids with this algebraic structure. An lcm-monoid $ M $ is a left-cancellative monoid such that $ 1 $ is the unique invertible element in $ M $, and every pair of elements in $ M $ admit an lcm with respect to left-divisibility. The class of lcm-monoids contains the Gaussian, quasi-Garside and Garside monoids.
Tom Quinn-Gregson - Minimum degrees of variants of full transformation semigroups ( )
Abstract. Every finite semigroup $ S $ embeds into some transformation semigroup $ \mathcal{T}_n $, and we call the least such $ n $ the (minimal transformation) degree of $ S $, denoted $ \mathrm{deg}(S) $. In this talk we examine how taking a variant affects the degree of $ S $, where for a fixed element $ a\in S $, the variant of $ S $ is the semigroup $ S^a=(S,\star) $ with sandwich product $ x\star y = xay $. By finding the degrees of null semigroups and right null semigroups (right zero semigroups of null semigroups), we are able to answer two questions of J. East from "Transformation Representations of Sandwich Semigroups". Namely, we show that it is possible to have $ \mu(S^a)< \mu(S) $ and deg $ (\mathcal{T}_n^\alpha) < 2n - \mathrm{rank}(\alpha) $.

This is joint work with P. Cameron, J. East, D. FitzGerald, J. Mitchell, and L. Pebody.
Stuart Margolis - Translational hulls, combinatorics and discrete geometry ( )
Abstract. In combinatorics and algebra morphisms are usually defined by preserving direct image: "A morphism between graphs (simplicial complexes) is a function that maps edges to edges (faces to faces), for example.

But in topology, morphisms are defined by preservation of inverse image of designated subsets, the open or closed sets.

More generally, an incidence structure is a pair $\mathcal{I}= (V,B) $ where $ V $ is a finite set and $ B $ is a collection of subsets of $ B $. The incidence matrix $ M(\mathcal{I}) $ is the $ V \times B $ matrix over $ \{0,1\} $ that has a 1 in position $ (v,b) $ if $ v \in b $ and $ 0 $ otherwise.

A partial function on $ V $ is called continuous if for each $ b \in B, bf^{-1} \in B $ whenever $ bf^{-1} $ is non-empty. Thus for the incidence structure consisting of a topolgical space $ V $ and its collection Bof open sets, the monoid of total continuous functions are the continuous functions on $ V $ in the usual sense.

The monoid $ C(\mathcal{I}) $ of continuous partial functions provides a link between combinatorics, discrete geometry and semigroup theory. It is easy to show that $ C(\mathcal{I}) $ is the translational hull of the 0-simple semigroup whose structure matrix is $ M(\mathcal{I}) $. This gives geometric and combinatorial tools to study translational hulls and their subsemigroups.

We illustrate this straightforward idea in the cases of graphs, simplicial complexes, Steiner triple systems and pairwise balanced designs.
Xavier Mary - Semigroups with chains of associate idempotents ( )
Abstract. It was discovered very recently that many classes of rings can interestingly be defined in terms of chains of associate idempotents. The purpose of this talk is to explore such connections in the semigroup case. Precisely, we will study non-regular semigroups with the property that any two $ \mathcal{D} $-related idempotents are related by a sequence of size $ n $ of alternatively $ \mathcal{L} $- or $ \mathcal{R} $- related idempotents. We will mainly focus on the cases $ n=1, 2 $ and $ 3 $. In the particular case of $ n=2 $, we obtain a semilattice decomposition in terms of extensions of completely simple semigroups by poor semigroups.
Carl-Fredrik Nyberg Brodda - Free products and word-hyperbolic monoids ( )
Abstract. Language-theoretic methods in group theory have been used successfully to characterise many different algebraic properties, including virtual freeness and hyperbolicity. In the past 20 years, there has been an increased effort to explore such methods in semigroup theory. A staple method for constructing new semigroups and monoids from old is by taking free products. Free products are well known to interact well with context-free languages in groups - for example, Anisimov proved that the free product of two groups with context-free word problem is again context-free, and it is easy to show that free products of hyperbolic groups are hyperbolic. In this talk, I will give an overview of how free products of semigroups and monoids can interact with formal language theory in the context of the word problem and word-hyperbolicity. Many existing techniques for studying this require some fixed implementation of the context-free languages (e.g. push-down automata). The methods used here instead use closure properties of the classes of languages involved, yielding much more general results. For example, we recover and generalise a recent result of Brough, Cain and Pfeiffer that the class of context-free monoids are closed under free products. Furthermore, new purely group-theoretic results can be obtained, e.g.: the free product of two groups with ET0L word problem again has ET0L word problem.
(Itamar Stein was also invited to speak but was sadly unable to attend; he has kindly made his slides available.)

31

The thirty-first NBSAN meeting was a "Young NBSAN" organised by and with a special focus on early career researchers. It was held in Manchester on 25th March 2022, organised by Thomas Aird, Daniel Heath, Dmitry Kudryavtsev, Carl-Fredrik Nyberg Brodda and Georgia Schneider. The speakers were:

Thomas Aird - Semigroup identities in tropical matrices
Ashley Clayton - Counting subdirect powers of finite commutative semigroups (slides)
Ambroise Grau - Translational hulls and isomorphisms with endomorphism monoids
Craig Miller - The $ \mathcal{R} $-height of semigroups and their bi-ideals (slides)
Nik Ruškuc - Direct and subdirect products in combinatorial semigroup theory (slides)
Georgia Schneider - Semigroups of inverse quotients (slides)
Maria Tsalakou - Computing congruences of finitely presented semigroups
Bill de Witt - The number of subdirect powers of unary algebras

In Summer 2021 we hosted some another short series of "eNBSAN" seminars:

14th July - James East - Congruences of twisted partition monoids ( )
Abstract. This is a report on a project with Nik Ruškuc, in which we classify the congruences of the twisted partition monoids, and describe the lattice formed by them.
21st July - Student Talks Session:
- Alex Levine - Describing solutions to equations using EDT0L languages ( )
  Abstract. Ever since Makanin proved that the satisfiability of systems equations in free monoids and groups was decidable, there has been significant interest in the study of equations in groups and semigroups. More recently, Ciobanu, Diekert and Elder proved that solutions to systems of equations in free monoids and free groups can be expressed as EDT0L languages. Following this there have been several attempts widen the class of groups and semigroups for which solutions can be expressed as an EDT0L language. We give a brief introduction to the work that has been done in this area.
- Georgia Schneider - Semigroups of left I-quotients ( )
  Abstract. A subsemigroup $ S $ of an inverse semigroup $ Q $ is a left I-order in $ Q $ if every element in $ Q $ can be written as $ a^{-1} b $ where $ a, b \in S $ and $ a^{-1} $ is the inverse of $ a $ in the sense of inverse semigroup theory. If, in addition, we can insist that $ a $ and $ b $ are $\mathcal{R}$-related in $ Q $, then we say that $ S $ is a straight left I-order in $ Q $. We give necessary and sufficient conditions for a semigroup to be a straight left I-order.
- Maria Tsalakou - Computational methods for small overlap monoids ( )
  Abstract. Small overlap monoids are monoids with finite presentations that satisfy simple combinatorial conditions. Finitely presented monoids that satisfy sufficiently strong small overlap conditions have decidable word problem and lexicographic normal forms that can be computed in linear time. We will give a brief description of the properties and the main results that led to the development of practical computational methods for small overlap monoids.
28th July - António Malheiro - Identities in plactic-like monoids: a crystal approach ( )
Abstract. The Plactic monoid, whose elements can be identified with semi-standard Young tableaux, has long been considered an important monoid. It admits several descriptions, one of them in terms of, so-called, Kashiwara crystals. Kashiwara crystals are certain recursively-constructed edge-colored directed graphs, built from a given crystal basis. The way in which crystals give rise to monoids can be described in purely combinatorial terms. In previous work, we modified this construction allowing us to characterize other monoids, referred to as plactic-like, using this approach.

The question of identities satisfied by plactic-like monoid is a subject of an active study. In this talk, we will present a brief report of the results in this area and show how crystals can be used to study them. Abstract text here

In September 2020 we hosted a series of online expository lectures by Ashley Clayton (St Andrews) on "Direct and Subdirect Products in Groups, Semigroups and Algebras". The series, which was aimed chiefly at PhD students, was generously supported by an extra grant from the London Mathematical Society. You can access:

Videos of the Lectures (on the LMS Youtube channel).
Ashley's Lecture Notes

Accompanying the lectures were a series of informal short talks by NBSAN PhD students:

Thomas Aird - Permutability of tropical matrices
Carl-Fredrik Nyberg Brodda - The word problem for special monoids
Scott Carson - Right ideal Howson semigroups
Bill De Witt - Residual finiteness of monounary algebras and their direct products
Luke Elliot - Automorphisms of the Brin-Thompson groups nV
Ambroise Grau - The translational hull of an "almost completely zero-simple" ideal in an independence algebra
Veronica Kelsey - Base sizes of permutation groups
Finn Smith - Computing finite semigroups

To conclude the series we held a special eNBSAN seminar on 7th October, following on from the topic of Ashley's lectures:

Wolfram Bentz - Direct and Subdirect Products in Groups, Semigroups and Algebras - Congruences of Direct Products

In Summer 2020, in place of physical meetings cancelled due to the COVID-19 pandemic, we held a series of online eNBSAN Seminars:

24th June - Igor Dolinka - The prefix membership problem for one-relator groups, and its semigroup-theoretical cousins ( | slides )
Abstract. In early 1930s, Wilhelm Magnus proved his famous and celebrated result that the word problem is decidable for all one-relator groups: given two group words $ u,v $ over the alphabet $ X $, there is an algorithm deciding whether $ u $ and $ v $ represent the same element of the group given by a presentation of the form $ \langle X \mid w=1 \rangle $. This result is based on another important theorem proved earlier by Magnus, the Freiheitssatz, which, roughly speaking, locates many free subgroups in one-relator groups. Later on, this inspired investigations of the word problem for other algebraic structures defined by a single relation. For example, in the 1960s Shirshov proved that the word problem is decidable for all one-relator Lie algebras. Surprisingly, the problem whether the word problem is decidable for all one-relator monoids is still open (although several important cases have been resolved by Adjan in 1966, and Adyan and Oganessyan in 1987).

An important intermediate class of algebraic structures lying between groups and monoids are that of inverse monoids. In 2001 Ivanov, Margolis and Meakin highlighted the importance of investigating one-relator inverse monoids by showing that the (conjectured) decidability of the word problem for one-relator special inverse monoids (the ones defined by a relation of the form $ w=1 $) would imply a positive solution of the word problem for all one-relator monoids. They also showed that, under a condition that is very familiar within the inverse semigroup theory realm, in the $ E $-unitary case, solving the word problem for the inverse monoid $ M $ given by the presentation $ \langle X \mid w=1 \rangle $ is equivalent to solving the prefix membership problem for the group $ G $ given by the same presentation. This problem asks whether there exists an algorithm deciding whether a given word $ u $ represents an element of the submonoid $ P_w $ of $ G $ generated by all prefixes of the relator word $ w $.

In this talk I will present several recent results, obtained in collaboration with Robert D. Gray (UEA Norwich), pertaining to the prefix membership problem for a class of one-relator groups -- and thus having implications for the word problem for special one-relator inverse monoids. These results will be formulated in terms of the classical constructions in combinatorial group theory: amalgamated free products and HNN-extensions.
1st July - Stuart Margolis - Degree 2 Transformation Semigroups as Continuous Maps on Graphs: Foundations and Structure ( | slides )
Abstract. We develop the theory of transformation semigroups that have degree 2, that is, act by partial functions on a finite set such that the inverse image of each point has at most two elements. We show that the graph of fibers of such an action gives a deep connection between semigroup theory and graph theory. It is known that the Krohn-Rhodes complexity of a degree 2 action is at most 2. We show that the monoid of continuous maps on a graph is the translational hull of an appropriate 0-simple semigroup. We show how group mapping semigroups can be considered as regular covers of their right letter mapping image and relate this to their graph of fibers. This is joint work with John Rhodes.
8th July - Yingying Feng - Min network of congruences on an inverse semigroup ( | slides )
Abstract. A congruence on an inverse semigroup S is determined uniquely by its kernel and trace. Denoting by $ \rho_k $ and $ \rho_t $ the least congruence on S having the same kernel and the same trace as $ \rho $, respectively, and denoting by $ \omega $ the universal congruence on S, we consider the sequence $ \omega, \omega_k, \omega_t, (\omega_k)_t, (\omega_t)_k, \cdots $

We call these congruences, together with the inclusion relation for congruences, the min network of congruences on S. The quotients $ \{S/\omega_k\}, \{S/\omega_t\}, \{S/(\omega_k)_t\}, \{S/(\omega_t)_k\}, \cdots $, as S runs over all inverse semigroups, form quasivarieties.

In this talk, I will talk about the repeated patterns in the resulting quotient semigroups. These patterns help us not only determine the quasivarieties to which the quotient semigroups belong, but also obtain relationships among these quasivarieties. This is joint work with Li-Min Wang, Lu Zhang, Hai-Yuan Huang and Zhi-Yong Zhou.
15th July - Robert Gray - Solving equations in one-relator monoids ( | slides )
Abstract. By the Diophantine problem we mean the algorithmic problem of determining if any given system of equations has a solution. Two classical results due to Makanin (1977, 1983) show that the Diophantine problem is decidable in any free monoid, and in any free group. It is natural to ask to what extent these results can be extended to classes of monoids and groups which are "close to being free", in some sense. In this talk I will present some results from joint work with Albert Garreta (Bilbao) in which we investigated this question for one-relator monoids.
22nd July - Benjamin Steinberg - Homological finiteness of one-relator monoids and related monoids ( | slides )
Abstract. The word problem for one-relator monoids is a longstanding open question. Kobayashi asked in 2000 whether every one-relator monoid admits a finite complete rewriting system. A necessary condition to have a finite complete rewriting system is to satisfy the homological finiteness condition $ FP_{\infty} $. Kobayashi also asked in 2000 whether every one-relator monoid is of type $ FP_{\infty} $ and proved that each one-relator monoid is of type $ FP_3 $.

In this talk, we discuss our proof that every one-relator monoid is indeed of type $ FP_{\infty} $. Our techniques combine methods from algebraic topology (monoids acting on CW complexes), homological algebra and the theory of monoid van Kampen diagrams. This is joint work with Bob Gray.
29th July - Marianne Johnson - Tropical representations and identities of plactic monoids ( | slides )
Abstract. I will report on some joint work with Mark Kambites (https://arxiv.org/abs/1906.03991). We show that the plactic monoid of every finite rank has a faithful representation by upper triangular matrices over the tropical semiring. This answers a question first posed by Izhakian and subsequently studied by several authors. A consequence is a proof of a conjecture of Kubat and Okniński that every plactic monoid of finite rank satisfies a non-trivial semigroup identity. In the converse direction, we show that every identity satisfied by the plactic monoid of rank $ n $ is satisfied by the monoid of $ n \times n $ upper triangular tropical matrices. In particular this implies that the variety generated by the $ 3 \times 3 $ upper triangular tropical matrices coincides with that generated by the plactic monoid of rank $ 3 $.

30

The thirtieth meeting was held in Manchester on 12th July 2019, organised by Marianne Johnson and Mark Kambites. The speakers were:

James East - Semigroups generated by idempotents and one-sided units (slides)
Bakh Khoussainov - Open questions on automatic structures (joint talk with Manchester Pure Mathematics Colloquium and LMS Aitken Lecture) (slides)
Finn Smith - Translational hulls of ideals (slides)
Nóra Szakács - Inverse monoids and immersions of cell complexes (slides)

29

The twenty-ninth meeting was held in York on 10th and 11th January 2019, organised by Victoria Gould. The speakers were:

Wolfram Bentz - The existential transversal property and its impact on the regularity of finite transformation semigroups
Matthew Brookes - The lattice of left congruences on an inverse semigroup
Scott Carson - The Howson property for one-sided ideals of a semigroup (slides)
Laura Ciobanu - Solving equations in semigroups and groups
Fernando Flores Brito - Congruences on End F_n(G) (slides)
Robert Gray - A Lyndon's identity theorem for one-relator monoids
Peter Hines - Card shuffles and Cantor space: an inverse semigroup perspective (slides)
Marianne Johnson - Tropical matrix fountains
Dandan Yang - Semigroups with finitely generated universal left congruence (slides)

28

The twenty-eighth NBSAN meeting was held in St Andrews on 14th and 15th June 2018, as a satellite meeting to the 2018 British Mathematical Colloquium. The organisers were Markus Pfeiffer and Nik Ruškuc. The speakers were:

Tara Brough - Two applications of monoid actions to cross-sections
Tom Coleman - Generation of intermediate and partial map monoids of first-order structures (slides)
James East - Sandwich semigroups in locally small categories
Justine Falque - The orbit algebra of oligomorphic permutation groups with polynomial profile - proof of a conjecture of Cameron (slides)
Florent Hivert - The 0-rook monoid and its friends (slides)
Marianne Johnson - Unitriangular matrix varieties
Itamar Stein - The global dimension of the algebra of the monoid of all partial functions on an n-set (slides)

27

A group photo taken at the 27th NBSAN.

The twenty-seventh meeting was held in York on 7th and 8th January 2018, organised by Victoria Gould and Rida-E Zenab, and in honour of the 70th birthday of László Márki. Above is a group photo. The speakers were:

Ashley Clayton - Subdirect products of free semigroups (slides)
Igor Dolinka - Universal locally finite maximally homogeneous semigroups (slides)
Victoria Gould - Coherency and purity for monoids (slides)
Tatiana Jajcayová - Inverse semigroups of partial automorphisms of combinatorial structures: report on recent progress
Valdis Laan - Morita equivalence for factorizable semigroups
László Márki - Morita equivalence of semigroups revisited: firm semigroups (slides)
John Meakin - Adian groups, semigroups and inverse semigroups
Craig Miller - Right noetherian semigroups
P. A. Azeef Muhammed - The cross-connection of the inductive groupoid of a regular semigroup (slides)
Pham Ngoc Ánh - A generalization of Clifford's theorem (slides)
Mária B. Szendrei - Extensions and I-semidirect products of inverse semigroups (slides)
Abdullahi Umar - Some remarks on semigroups of contraction mappings of a finite chain (slides)
Rida-E Zenab - Pseudo-finite monoids and semigroups (slides)
Xia Zhang - On injective constructions of S-semigroups

26

The twenty-sixth NBSAN meeting was held in St Andrews on 13th July 2017, organised by Julius Jonusas. The speakers were:

Nick Gilbert - Extensions and cohomology of inverse semigroups (slides)
Zur Izhakian - Tropical plactic algebra and semigroup representations
Mark Kambites - A random walk through random walks (slides)
Matt McDevitt - Insertion relations on words
Munazza Naz - Extensions of Green's relations on semigroups of tropical matrices (slides)

25

The twenty-fifth NBSAN meeting took place in York on Wednesday 18th January 2017, organised by Victoria Gould and incorporating a meeting of the York Semigroup. The speakers were:

Wolfram Bentz (Hull) - Congruences on the product of two full transformation monoids (abstract | slides)
Tara Brough (Lisbon) - Word problems of free inverse monoids (abstract)
Julius Jonusas (St Andrews) - Universal words and sequences (abstract | slides)
Emma McDougall (Heriot-Watt) - Relation modules for inverse monoids
Stuart Margolis (Bar Ilan) - Projective indecomposable modules and quivers for a class of Fountain monoids with applications to descent algebras (abstract)
Peter Neumann (Oxford) - Galois and his groups (York Semigroup Talk, abstract)

24

The twenty-fourth NBSAN meeting was held in York on Wednesday 11th May 2016, organised by Victoria Gould and with a focus on semigroup algebras and connections with functional analysis. The speakers were:

Garth Dales (Lancaster) - Ideals in βS (slides)
Robert Gray (East Anglia) - Cellular and standardly based semigroup algebras (slides)
Chris Hollings (Oxford) - A brief history of semigroup representations (slides)
Mark Kambites (Manchester) - Some conditions related to amenability
Catarina Santa-Clara (Lisbon) - The semigroup ring of a restriction semigroup with an inverse skeleton (slides)
Dona Strauss (Leeds) - Idempotents in βS (slides)

23

The twenty-third NBSAN meeting was held in St Andrews on Thursday 17th and Friday 18th March 2016, organised by Markus Pfeiffer. The speakers were:

Ying-Ying Feng (Foshan and York) - Two kinds of congruence networks on regular semigroups (slides)
Marianne Johnson (Manchester) - Face monoid actions and tropical hyperplane arrangements
Mark-Jan Nederhof (St Andrews) - A short proof that O_2 is an MCFL (slides)
Michael Torpey (St Andrews) - Computing with semigroup congruences (slides)
Rebecca Waldecker (Martin Luther Halle-Wittenberg) - Applying permutation group theory
Wilf Wilson (St Andrews) - Computing maximal subsemigroups of finite semigroups (slides)

22

The twenty-second NBSAN meeting took place in Manchester on Friday 20th November 2015, organised by Mark Kambites and Matthew Taylor. The meeting was held in conjunction with the First CoDiMa Training School in Computational Discrete Mathematics, and the morning of the NBSAN meeting was a featured an introduction to Semigroups in GAP given by James Mitchell. The speakers were:

Peter Fenner (Manchester) - The gossip monoid
Maximilien Gadouleau (Durham) - Universal simulation of automata networks (slides)
Stuart Margolis (Bar Ilan) - Poset cohomology, CW decompositions and the global dimension of left regular band algebras (slides)
James Mitchell (St Andrews) - Semigroups in GAP: theory and practice (joint session with the CoDiMa training school)
Colva Roney-Dougal (St Andrews) - Relations relating to generation of groups and semigroups

21

The twenty-first NBSAN meeting was held at the University of Essex, in Colchester, on 10th July 2015, organised by Peter Higgins, Alexei Vernitski and Gerald Williams. The speakers were:

Tara Brough - Automaton semigroups: some new examples and non-examples
Jack Button (Cambridge) - A summary of recent results on 1-relator groups (abstract)
Andrew Duncan (Newcastle) - Equations over hyperbolic groups and semigroups (abstract)
Nick Loughlin (Newcastle) - Understanding idempotents in diagram semigroups and algebras (abstract)
Esamaldeen Mohamed (Essex) - Isomorphisms amongst cyclically presented groups (abstract)
Jim Renshaw (Southampton) - Free products and amalgams of pomonoids (abstract)

20

The twentieth NBSAN meeting was held in St Andrews on 22nd and 23rd April 2015, organised by Markus Pfeiffer. Peter Cameron wrote a very nice blog post on the meeting. The speakers were:

Paul Bell (Loughborough) - Complexity of reachability, mortality and freeness problems for matrix semigroups (slides)
Tom Bourne (St Andrews) - Tackling the generalised star-height problem (slides)
Alan Cain (Universidade Nova de Lisboa) - Computations and conjugacy in hypoplactic and sylvester monoids and other homogenous monoids (slides)
Peter Cameron (St Andrews) - Finding where you are: automata, graph endomorphisms and foldings (slides)
Rachael Carey (St Andrews) - Graph automatic semigroups (slides)
Robert Gray (East Anglia) - Crystal monoids (slides)
Ruth Hoffmann (St Andrews) - Geometric grid permutation classes and regular languages
Matthew Taylor (Manchester) - A classification of 2-generated T-modules

19

The nineteenth NBSAN meeting was held in York on 14th January 2015, organised by Victoria Gould. Speakers were:

Lovkush Agarwal (Leeds) - The reducts of the generic digraph
Bernard Bainson (Heriot-Watt) - Khovanov's presheaf on some ordered groupoids (slides)
Wolfram Bentz (Lisbon) - New results on synchronization
Igor Dolinka (Novi Sad) - Variants of semigroups - the case study of finite full transformation monoids (slides)
Rowena Paget (Kent) - A representation theory approach to the rook monoid (slides)
Goncalo Pinto (College of the Bahamas) - Residuated completely simple semigroups (slides)
Tom Quinn-Gregson (York) - ℵ₀-categorical semigroups (slides)
Itamar Stein (Bar-Ilan) - The ordinary quiver of the algebra of the monoid of all partial functions on a set (slides)

18

The eighteeth NBSAN meeting was held in Edinburgh on Monday 21st July 2014, organised by Nick Gilbert. The speakers were:

Karl Auinger (Vienna) - On the membership problem for pseudovarieties (slides)
James East (Western Sydney) - Idempotent generators in diagram semigroups (slides)
Ganna Kudryavtseva (IMFM Ljubljana) - On non-commutative frame theory (slides)
Laurent Poinsot (Paris Nord XIII) - From combinatorial monoids to algebras, bialgebras and Hopf algebras, functorially (slides)

17

The seventeenth NBSAN meeting was held in Norwich on Monday 14th and Tuesday 15th April 2014, organised by Robert Gray. The speakers were:

Catarina Carvalho (Hertfordshire) - Constraint satisfaction problems and dualities (slides)
Cong Chen (Leeds) - The word norm
Igor Dolinka (Novi Sad) - Free idempotent generated semigroups: maximal subgroups and the word problem (slides)
Jan Foniok (Warwick) - Constraint satisfaction problems with tree duality (slides)
Peter Higgins (Essex) - Languages that require full scanning of words to determine membership (slides)
Rick Thomas (Leicester) - Word problems of groups, formal languages and decidability (slides)
Alexei Vernitski (Essex) - From knot groups to knot semigroups (slides)
Dandan Yang (York) - Free idempotent generated semigroups and endomorphisms (slides)

16

The sixteenth NBSAN meeting was held in York on Wednesday 20th November 2013, organised by Victoria Gould. The speakers were:

Jenni Awang - Semigroups, Cayley graphs and finite presentation (abstract)
Alan Cain - Endomorphisms of semigroups: growth and interactions with subsemigroups (abstract | slides)
Robert Gray - Ideals and finiteness conditions for subsemigroups (abstract)
Valdis Laan - Congruence extension property for ordered algebras (abstract | slides)
John Truss - Some monoids and notions of homomorphism-homogeneity (abstract)
Yanhui Wang - Left restriction semigroups from incomplete automata (abstract | slides)

15

The fifteenth NBSAN meeting was held in Manchester on Wednesday 24th July 2013, organised by Marianne Johnson and Mark Kambites. The speakers were:

Alex Bailey - Subsemigroup growth of finitely generated free semigroups (slides)
John Meldrum - Some aspects of endormorphism semigroups
Markus Pfeiffer - Deciding word problems with finite state devices
Uday Reddy - Transformation monoids in programming language semantics (slides)
Alistair Wallis - K-theory of inverse semigroups (slides)
Rida-E Zenab - Semigroups with skeletons and Zappa-Szép products (slides)

14

The fourteenth NBSAN meeting was held in St Andrews on Tuesday 9th April 2013, organised by Yann Peresse. The speakers were:

Pham Ngoc Ánh - Divisibility, ideal and valuation theory of classical commutative rings
Victoria Gould - Coherent monoids (slides)
James Hyde - Sierpinski rank and universal sequences (slides)
Julius Jonusas - Maximal subsemigroups of the transformation monoid
Max Neunhöffer - Generalisations of small cancellation (slides)
David Wilding - Green's D relation for matrices over semirings

13

The thirteenth NBSAN meeting was held in York on Wednesday 21st November 2012, organised by Victoria Gould. The speakers were:

Igor Dolinka (Novi Sad) - Finite groups are big as semigroups (slides)
David Janin (Bordeaux) - Walking in free inverse monoids (slides)
Peter Jones (Marquette) - The semigroups B_2 and B_0 are inherently nonfinitely based (slides)
Alex McLeman (St Andrews) - Cayley automaton semigroups (slides)
Dandan Yang (York) - Free idempotent-generated semigroups (slides)

12

The twelfth NBSAN meeting (which was the first Young NBSAN meeting) was held at ICMS in Edinburgh on Monday 23rd July 2012, organised by Alex Bailey, Alistair Wallis and Yanhui Wang. The speakers were....

Suhear Alwan (Essex) - Languages that require full scanning of words to determine membership
Tara Brough (St Andrews) - Automaton semigroup constructions
Zsofia Juhasz (Essex) - Congruences induced by Green's relations and the corresponding egg-box pictures
Mark Kambites (Manchester) - An introduction to the Cerny conjecture (expository talk)
Bana Al Subaiei (Southampton) - Amalgamation in pomonoids and dominions
Alina Vdovina (Newcastle) - Short introduction to expander graphs

11

The eleventh NBSAN meeting was held in Manchester on Wednesday 11th April 2012. The speakers were:

Des FitzGerald (Tasmania) - Categorification for binary relations (abstract | notes)
Miklos Hartmann (York) - E-unitary almost factorizable orthodox semigroups (slides)
Peter Higgins (Essex) - Products in transformation semigroups: work with the late John M. Howie (slides)
Sarah Rees (Newcastle) - Artin groups and monoids; normal forms and the group word problem (slides)

10

The tenth NBSAN meeting was held in York on Wednesday 23rd November 2011. The speakers were:

Peter Cameron - Permutation groups and transformation semigroups (abstract | slides)
Jean-Eric Pin - Expansions and covers (abstract | slides)
Rick Thomas - The word problem for semigroups (abstract | slides)
Yanhui Wang - Weakly U-abundant semigroups (abstract | slides)

9

The ninth NBSAN meeting was held in Manchester on Tuesday 30th August 2011. The speakers were:

Alex Bailey (Southampton) - Flat covers of acts over monoids
Victoria Gould (York) - Restriction semigroups (an expository talk) (slides)
Robert Gray (Lisbon) - Structures with lots of symmetry (handout slides | overlay slides)
Christopher Hollings (Oxford) - The perils of taking shortcuts: embedding semigroups in the 1930s (slides)
Lubna Shaheen (York) - Model companions of S-posets

8

The eighth NBSAN meeting was held at the University of St Andrews on Thursday 19th and Friday 20th May 2011. The speakers were:

Alan Cain (Porto) - Hyperbolic and word-hyperbolic semigroups (slides)
Si Craik (St Andrews) - Ends of semigroups (slides)
Andrew Duncan (Newcastle) - Rewriting systems and the geodesic problem for words and cycles
John Fountain (York) - An inverse monoid approach to Thompson's group V and generalisations (slides)
Marianne Johnson (Manchester) - Green's J-order and the rank of tropical matrices (slides)
Dave Jones (Heriot Watt) - Graph Inverse Semigroups (slides)
Mark Kambites (Manchester) - An Introduction to Tropical Matrix Semigroups
Alistair Wallis (Heriot Watt) - Rees monoids, self-similar groups and fractals (slides)

7

The seventh NBSAN meeting was held at the University of York on Wednesday 24th November 2010. The speakers were:

Bassima Afara (Heriot-Watt) - The Rees theorem and its generalizations (slides)
Peter Jones (Marquette) - Varieties of P-restriction semigroups (slides)
Volodymyr Mazorchuk (Uppsala) - Kiselman type semigroups
Maria Szendrei (Szeged) - Some results on almost factorisable semigroups (slides)
Alexei Vernitski (Essex) - Graphs to semigroups (slides)

6

The sixth NBSAN meeting was held at the University of Manchester on Monday 26th July 2010. The speakers were:

Zur Izhakian (Bar Ilan) - Supertropical algebra
Erzsi Dombi (Glasgow) - Automatic semigroup acts
Rick Thomas (Leicester) - FA-presentable semigroups

5

The fifth NBSAN meeting was held at the University of St Andrews on Friday 7th and Saturday 8th May 2010, organised by James Mitchell. Speakers were:

Andreas Distler (St Andrews) - How to store 10 semigroups in a bit
James East (Sydney) - Generators and relations for monoids of block bijections and partitions
Attila Egri-Nagy (Hertfordshire) - Coordinatizing finite permutation groups and transformations semigroups - computational considerations
Victoria Gould (York) - Bisimple inverse semigroups as semigroups of quotients
Zsofia Juhasz (Essex) - The smallest operation-compatible quasiorders containing Green's quasiorders
Zak Mesyan (Ben Gurion) - Conjugation of injections by permutations
Markus Pfeiffer (St Andrews) - Semigroups with easily solveable word problem
Steve Pride (Glasgow) - On diagram groups
Abdullahi Umar (Sultan Qaboos) - Some combinatorial properties of semigroups of partial transformations

4

The fourth NBSAN meeting was held at the University of York on Wednesday 25th November 2009, organised by Victoria Gould. Speakers were:

Claire Cornock (York) - Structure theorems for proper restriction semigroups
Nick Gilbert (Heriot-Watt) - Approaches to tiling semigroups
Michael Hoffmann (Leicester) - Hyperbolic monoids
Marianne Johnson (Manchester) - Tropical matrix algebra
Jim Renshaw (Southampton) - Adequate transversals of abundant semigroups (abstract | slides)
Lubna Shaheen (York) - Perfection for pomonoids

3

The third NBSAN meeting was held on 23rd July 2009 at Heriot-Watt University in Edinburgh, organised by Mark Lawson. The speakers were:

Chris Eilbeck (Heriot-Watt) - Weierstrass arithmetic semigroups and algebraic curves
Jennie Hansen (Heriot-Watt) - Using probability theory to study the full transformation monoid
Peter Higgins (Essex) - Set products in transformation semigroups
Stuart Margolis (Bar-Ilan) - On some monoids associated to Coxeter groups
James Mitchell (St Andrews) - Generating uncountable transformation semigroups

2

The second NBSAN meeting was held in St Andrews on 16th and 17th April 2009. The speakers were:

João Araújo (Lisbon) - Guess Who
Alan Cain (Lisbon) - Automaton Semigroups
Robert Gray (St Andrews) - Homotopical and Homological Finiteness Conditions of Monoids and their Subgroups
Mark Kambites (Manchester) - Free Adequate Semigroups
Vesna Kilibarda (Indiana Northwest) - Ends for Semigroups and Monoids
Valdis Laan (Tartu) - Tensor Products and Preservation of Weighted Limits for S-posets
Mark Lawson (Heriot-Watt, expository talk) - The Morita Theory of Regular Semigroups
László Márki (Rényi Institute) - A Matrix Construction for certain Inverse Monoids with an Application in Universal Algebra

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The inaugural NBSAN meeting was held in York on 28th January 2009, and was dedicated to the memory of Professor Douglas Munn (1929-2008). The speakers were:

John Fountain (York, expository talk) - The Mathematical Work of Douglas Munn and its Influence
Nik Ruškuc (St Andrews, open problem talk) - One Relation Semigroups
Rebecca Noonan Heale (Heriot-Watt) - Green's Relations for Semigroup Graph Expansions
Yann H. Peresse (St Andrews) - Generating Transformation Semigroups using Endomorphisms of Preorders, Graphs and Similarity Relations
Pedro V. Silva (Porto) - The Generalized Conjugacy Problem for Virtually Free Groups
Jorge Almeida (Porto) - The Mysteries of Free Profinite Semigroups: a Sneak Preview