13th meeting: Groups & Geometry

SUBJECT TO THE LMS AWARDING FUNDING TO THE “TRIANGLE” NEXT YEAR…

Birkbeck, University of London, Wednesday December 13th 2017

Room B3 of 43 Gordon Square. Note that this is not the usual building but is nearby (if you’ve walked to Birkbeck from somewhere like Euston or King’s Cross before then you’ve practically walked past it. You’re warned that an accurate map of the building itself can be found here. If you would like some directions then email b.fairbairn[usual funny AT symbol]bbk.ac.uk . Afterwards we will very likely go The Bree Louise followed by a curry (assuming they’re still standing.)

Owing to exceptional budget constraints we politely request that any postgraduate who will be requesting that we cover their travel expenses seek-out the cheapest means of getting here possible (typically a pre-booked train is fine).

13:30-14:30 Emilio Pierro (LSE) TBA

TBA

14:30-15:30 Henry Bradford (Göttingen) TBA

TBA

15:30-16:00 Break

16:00-17:00 Caroline Series (Warwick) TBA

TBA

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12th meeting: Permutation Groups

University of Bristol, May 17th 2017

School of Mathematics, University Walk, lectures will take place in room SM2.

After the last talk we will go for drinks and dinner.

Owing to exceptional budget constraints we politely request that any postgraduate who will be requesting that we cover their travel expenses seek-out the cheapest means of getting here possible (typically a pre-booked train is fine).

14:00 – 15:00 Joanna Fawcett (University of Cambridge) Finite k-connected-homogeneous graphs

A graph is homogeneous if any isomorphism between finite induced subgraphs extends to an automorphism of the entire graph. The finite homogeneous graphs have been completely classified, and only a few families of examples arise. In this talk, we will discuss several methods of relaxing the hypothesis of homogeneity. In particular, we will discuss some recent progress on classifying the finite k-connected-homogeneous graphs where k>3. This is joint work with A. Devillers, C.H. Li, C.E. Praeger and J.-X. Zhou.

15:00 – 15:30 Break

15:30 – 16:30 Melissa Lee (Imperial College) Bases of quasisimple linear groups and Pyber’s conjecture

A base of a group G acting faithfully on a set  Ω is a subset B⊆Ω such that the pointwise stabiliser of B in G is trivial. The minimal base size of G is denoted by b(G).

A well-known conjecture made by Pyber in 1993 states that there is an absolute constant c such that if G acts primitively on Ω, then b(G) < clog|G|/log n, where |Ω|=n.

Following the contributions of several authors, the conjecture was finally established in 2016 by Duyan, Halasi and Maróti.

A result that played a major role in the proof of Pyber’s conjecture for primitive linear groups was given by Liebeck and Shalev, who proved that there is a constant C such that if G is a quasisimple group acting irreducibly on a finite vector space V, then either b(G)C, or G is an alternating or classical group acting on its natural module. In this talk, I will cover the history of Pyber’s conjecture, especially in the context of primitive linear groups, and present some results on the determination of the constant C for bases of quasisimple groups. I will also discuss an application by Liebeck of the latter result, which improves the known upper bounds for b(G) when G is an irreducible primitive linear group.

16:30 – 17:30 Simon Smith (University of Lincoln) The box product of two permutation groups

There are a number of ways in which one may take the product of two groups. Products which possess some kind of “universal” property (like the free and wreath products), or those which preserve some of the important properties of the input groups, are rare and precious.

Arguably, the most important product in permutation group theory is the wreath product, acting in its product action. The reason for this is that, unlike other products, it preserves a fundamental property called primitivity. Primitive permutation groups are indecomposable in some sense, and for finite groups they are the basic building blocks from which all permutation groups are comprised.

I am going to talk about a new product, called the box product. It is fundamentally different to the wreath product in product action. Nevertheless, it preserves primitivity under astonishingly similar conditions. Moreover, the box product has a “universal” property, and under natural conditions on groups M and N, the product of M and N is simple. The product can be used to easily solve a well-known open problem from topological group theory, and has an important role to play in the classification of infinite permutation groups.

11th meeting: Finite Groups

University of Manchester, Wednesday March 1st 2017

Owing to exceptional budget constraints we politely request that any postgraduate who will be requesting that we cover their travel expenses seek-out the cheapest means of getting here possible (typically a pre-booked train is fine).

All talks will be held in Room G205 of the Alan Turing Building

1.30 – 2.20 Sarah Hart (Birkbeck)  Product-free sets and Filled Groups

2.30 – 3.20 Ali Aubad (Manchester) Commuting Involution Graphs for Double Covers of the Symmetric Groups

3.30 – 4.00   Refreshments

4.00-4.50 David Ward (Manchester) Cuspidal Character and Finite Sporadic Simple Groups

The last talk will be followed by going for further refreshment followed by a curry.

10th meeting: Groups & Representations

University of Birmingham, Wednesday January 25th 2017

All lectures will take place Lecture Room B of the Watson Building.

Owing to exceptional budget constraints we politely request that any postgraduate who will be requesting that we cover their travel expenses seek-out the cheapest means of getting here possible (typically a pre-booked train is fine).

14:00-15:00 Carolina Vallejo Rodríguez (ICMAT) Detecting local properties in the character table

Let G be a finite group and let p be a prime number. In this talk, we discuss local properties of G that can be read off from its character table. More precisely, we characterize globally when the principal block of the normalizer of a Sylow p-subgroup has one simple module for p odd. We also talk about the p=2 case of this problem, which remains open. This is joint work with G. Navarro and P. H. Tiep.

15:00-16:00 Eugenio Gianelli (Cambridge) Characters of odd degree of symmetric groups

Let G be a finite group and let P be a Sylow p-subgroup of G.

Denote by Irrp(G) the set consisting of all irreducible characters of G of degree coprime to p.

The McKay Conjecture asserts that |Irrp(G)|=|Irrp(NG(P))|.

Sometimes, we do not only have the above equality, but it is also possible to determine explicit natural bijections (McKay bijections) between Irrp(G) and Irrp(NG(P)).

In the first part of this talk I will describe the construction of McKay bijections for symmetric groups at the prime p=2.

In the second part of the talk I will present a recent joint work with Kleshchev, Navarro and Tiep, concerning the construction of natural bijections between IIrrp(G) and Irrp(H) for various classes of finite groups G and corresponding subgroups H of odd index. This includes the case G=Sn and H any maximal subgroup of odd index in Sn, as well as the construction of McKay bijections for solvable and general linear groups.

16:00-16:30 Break

16:30-17:30 Geoff Robinson (Aberdeen/Lancaster) On a subgroup introduced by J.Grodal

(report on ongoing joint work with J. Grodal).  We will discuss the structure of the (normal) subgroup of a finite group G generated by the elements whose centraliser has order divisible by the prime p.  This leads quickly to a study of an interesting generalization of Frobenius complements.

The abelianization of the associated quotient group plays a role in J. Grodal’s work on endotrivial modules.

9th meeting: Groups & Combinatorics

Birkbeck, University of London, Wednesday December 14th 2016

All talks will be held in room B04 of the main Malet Street building. Directions available on request from b.fairbairn [at ] bbk.ac.uk if needed.

Owing to exceptional budget constraints we politely request that any postgraduate who will be requesting that we cover their travel expenses seek-out the cheapest means of getting here possible (typically a pre-booked train is fine).

13:30-14:30 Atiqa Sheikh (Imperial) Orbital diameters of the alternating and projective special linear groups

In this talk we explore a particular family of edge-transitive graphs called orbital graphs. The orbital diameter of a primitive group G acting on a finite set Ω is defined to be the maximum of the diameters of the orbital graphs of G. We will give an overview of the results obtained on the orbital diameters of groups whose socle is an alternating group or a projective special linear group of dimension 2. The results concern boundedness of the orbital diameters and classifying orbital graphs of small diameters.

14:30-15:30 Jozef Širáň (OU) Regular maps arising from twisted linear groups

A map a cellular embedding of a graph on a surface. A flag of a map is a triple consisting of a vertex, an edge and a face side that are mutually incident. An automorphism of a map is a permutation of its flags such that two flags sharing a vertex, edge or a face are sent to a pair of flags sharing the same type of elements. The collection of all such mappings under composition forms the automorphism group of a map, acting semi-regularly on the flag set. If this action is regular we say that the map is regular as well. Regular maps may be viewed as having the ‘highest level of symmetry’ among all maps. A slightly weaker concept is that of an orientably-regular map, where only orientable surfaces and orientation-preserving automorphisms are considered.

Regular maps embody three varieties of objects: a graph, a surface on which the graph is embedded, and a group isomorphic to the automorphism group of the embedding. Classification of regular and orientably-regular maps, which is a central problem in the theory of symmetric maps, is therefore usually attempted for a particular collection of underlying graphs, or carrier surfaces, or automorphism groups.

In the talk we will focus on classification of regular and orientably-regular maps with a given (isomorphism type of) automorphism group. We will give a survey of available results; as it turns out, two-dimensional linear groups
PSL(2,q) and PGL2,q) appear to be best understood from this point of view. Interestingly, the PGL(2,q) groups are one of the two infinite families of finite sharply 3-transitive permutation groups. Their ‘counterpart’, however, i.e., the ‘other’ such infinite family consisting of the so-called twisted linear groups, have been somewhat neglected in the literature. We will also present new results on enumeration of regular and orientably-regular maps with twisted linear automorphism groups and discuss open questions in this area of research.

15:-16:00 Break

16:00-17:00 William Norledge (Newcastle) Covering Theory of Combinatorial Buildings

Buildings are certain simply connected and highly symmetrical polyhedral complexes, which have a combinatorial description as a “Weyl-metric space”. Towards a greater understanding of locally compact groups, the groups which act on buildings are studied in a hope to extend the work of Bruhat-Tits on algebraic groups over non-Archimedean local fields acting on Euclidean buildings.
Covering theory of complexes of groups has been used to construct actions of groups on buildings. During the talk, we’ll introduce covering theory of buildings which doesn’t resort to complexes of groups, but rather deals directly with the Weyl-metric. We’ll use this covering theory to construct actions of amalgams of surface groups on hyperbolic buildings.

8th meeting: Permutation Groups

University of Bristol, Wednesday June 1st, 2016

School of Mathematics, University Walk, lectures will take place in room SM1.

After the last talk we will go for drinks and dinner.

2:00-3:00 Gareth Tracey (Warwick) Generating minimally transitive permutation groups: a conjecture of Pyber

Suppose that G is a transitive permutation group, of degree n, but that G needs a large number of generators (in terms of n). If possible, we would like to “reduce” the number of generators, whilst keeping our group transitive. More precisely, we would like to take a subset X of G, minimal with the property that <X> is transitive. The question is: can we find a good upper bound for |X|, in terms of n? In this talk, we discuss the history of this question, including an old conjecture of Pyber, and some new results. We will also briefly describe some of the applications of the question.

3:00-3:30 Break

3:30-4:30 Peter Cameron (St Andrews) Road colouring and road closures: synchronization and idempotent generation

In recent years it has been noticed that knowledge about permutation groups can be applied to many old problems about transformation semigroups, especially considering regularity and idempotent generation. I have been working on this with Joao Araujo (Lisbon) and others. I will talk about two aspects of this.

A common type of problem asks what properties of a permutation group G force the semigroup generated by G and a non-permutation $a$ to have a particular property, for all or some specified elements a. Recently, we have considered the question of which permutation groups G have the property that, for any rank 2 map a, the semigroup <G,a> is idempotent generated. This condition forces G to be primitive, and is equivalent to the following property of G: for any orbit O of G on 2-sets, and any block B of imprimitivity for G acting on O, the graph with edge set O\B is connected. Such a group must be basic. It fails if G has an imprimitive subgroup of index 2, and it also fails for a class of primitive groups arising from triality. We conjecture, backed by substantial computational evidence, that it is true for all other primitive groups.

The other part of my talk will be an update on the ongoing synchronization project, which asks: which permutation groups G have the property that, for any non-permutation a, the semigroup <G,a> contains an element of rank 1.

4:30-5:30 Barbara Baumeister (Bielefeld) Covering a group by conjugates of a coset

For every doubly transitive permutation group G the conjugates
of a non-⁠trivial coset of a point stabilizer H cover the group.
This implies that every non-trivial conjugacy class of elements
of G that contains an element of H does contain a transversal
of H in G and that every other non-trivial conjugacy class
contains a transversal for the set of cosets of H in G different
from H.

We study the finite groups satisfying this property; more precisely
the class of primitive permutation groups, called
CCI groups, that in fact properly contains the class of 2-⁠transitive
permutation groups.

 

7th meeting: Finite Groups

University of Manchester, Wednesday March 9th, 2016

All talks will be held in room G108 of the the Alan Turing building. The talks will be followed by “the usual pilgramage to the pub and curry place.”

1:30-2:20 Peter Rowley (Manchester) An algorithm for the Thompson subgroup of a p-group

2:30-3:20 Madeleine Whybrow (Imperial) Majorana Representations of Triangle-Point Groups

Majorana Theory was introduced by A.A. Ivanov in 2009 as the axiomatisation of certain properties of the 2A-axial vectors of the 196,884-dimensional Monster Algebra. Ivanov’s work was inspired by a result of S. Sakuma which reproved certain important properties of the Monster Algebra in the context of Vertex Operator Algebras. Majorana Theory takes the key hypotheses of Sakuma’s result to provide a powerful framework, independent of Vertex Operator Algebras, in which to study the Monster Algebra and other related objects.

 

In this talk, I will discuss the history and motivation behind Majorana Theory before presenting my own work on Majorana Representations of Triangle-Point Groups. These are 6-transposition groups which are generated by 3 involutions, two of whom commute. They play an important role in the proof of the uniqueness of the Monster Group and in the study of the Monster Graph.

3:30-4:00 Break

4:00-4:50 Gernot Stroth (Halle) Groups which are almost groups of Lie type in characteristic p