December 2012

Tuesday 04 
Groups and Combinatorics Seminar
Neil Gillespie (UWA)
will speak on
Completely regular codes with large minimum distance
and
Daniel Hawtin (UWA) will speak on
Elusive Codes in Hamming Graphs
at 1pm Tuesday 4th of December in MLR2
Abstracts:
Completely regular codes with large minimum distance:
In 1973 Delsarte introduced completely regular codes as a generalisation of perfect codes. Not only are completely regular codes of interest to coding theorists due to their nice regularity properties, but they also characterise certain families of distance regular graphs. Although no complete classification of these codes is known, there have been several attempts to classify various subfamilies. For example, Borges, Rifa and Zinoviev classified all binary nonantipodal completely regular codes. Similarly, in joint work with Praeger, we characterised particular families of completely regular codes by their length and minimum distance, and additionally with Giudici, we also classified a family of completely transitive codes, which are necessarily completely regular. In this work with Praeger, and also with Giudici, the classification given by Borges, Rifa and Zinoviev was critical to the final result. However, recently Rifa and Zinoviev constructed an infinite family of nonantipodal completely regular codes that does not appear in their classification. This, in particular, led to a degree of uncertainty about the results with Praeger and with Giudici. In this talk I demonstrate how I overcame this uncertainty by classifying all binary completely regular codes of length m and minimum distance $ elta$ such that $ elta>m/2$.
Elusive Codes in Hamming Graphs:
We consider a code to be a subset of the vertex set of a Hamming
graph. We examine elusive pairs, codegroup pairs where the code is not
determined by knowledge of its set of neighbours. We provide an
infinite family of elusive pairs, where the group in question acts transitively
on the set of neighbours of the code. In our examples, we find that the
alphabet size always divides the length of the code, and prove
that there is no elusive pair for the smallest set of parameters for which this
is not the case.


February 2013

Friday 22 
15:00  SEMINAR  Groups and Combinatorics Seminar: Algebraic aspects of Hadamard matrices

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Our Groups and Combinatorics Seminar will resume this Friday.
Padraig Ó Catháin (The University of Queensland)
will speak on
Algebraic aspects of Hadamard matrices
at 3pm Friday 22nd of February in MLR2.
Abstract:
Hadamard matrices have applications in the design of experiments, signal processing, coding theory and many other areas. They have been extensively studied for many years, and are known to be closely related to symmetric designs with certain parameters. Many constructions for Hadamard matrices are known. Some are combinatorial in nature, others make use of finite fields and tools from abstract algebra.
In this talk I will give an introduction to Hadamard matrices, their automorphism groups, and their relations to other combinatorial objects. As a corollary of the classification of finite doubly transitive permutation groups, a classification of 'highly symmetric' Hadamard matrices is obtained. I will also look at the problem of constructing Hadamard matrices with primitive automorphism groups.


March 2013

Friday 01 
15:00  SEMINAR  Groups and Combinatorics Seminar, The Wall and Guralnick conjectures: history and legacy

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Abstract:
In 1961 G.E. Wall conjectured that the number of maximal subgroups of a finite group is less than the order of the group. The conjecture holds for all finite solvable groups (proved by Wall himself in his original paper) and holds for almost all finite simple groups, possibly all of them (proved by Liebeck, Pyber and Shalev in 2007). It is now known to be false in general, at least as originally stated, with infinitely many negative composite group examples found through a combination of computational and theoretical techniques. (I cite in particular computer calculations of Frank Luebeck, as partly inspired and later confirmed by calculations of my undergraduate student, Tim Sprowl, with theoretical input from myself and Bob Guralnick.) In this talk I will try to discuss the ingredients in this quite remarkable story, and I will mention as much of the legacy of positive consequences as time permits.

Tuesday 05 
13:00  SEMINAR  Groups and Combinatorics Seminar, Control of fusions in fusion systems and applications

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Jiping Zhang (Peking University)
will speak on
Control of fusions in fusion systems and applications
at 1pm on Tuesday 5th of March, in MLR2
Abstract:
Fusion systems were introduced by L. Puig in early 1990's mainly for the purpose of block theory. Fusion systems are also of interest in homotopy theory. In this talk we will define a new control of fusion in fusion systems and apply it to the study of maximal Sylow intersections.

Friday 08 
15:00  SEMINAR  Groups and Combinatorics Seminar, Generalised ngons and the FeitHigman theorem

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Name: Jon Xu (University of Melbourne/University of Western Australia)
will speak on
Generalised ngons and the FeitHigman theorem
at 3pm on Friday 8th of March.
Abstract:
Jacques Tits' theory of buildings played a vital role in the proof of the classification theorem on finite simple groups. The class of rank 2 buildings are also known as generalised ngons.
In my talk, generalised ngons will be defined as a certain class of bipartite graphs, so as to skip the (rather abstruse) buildingtheoretic definition. I will also state and outline a proof of the FeitHigman theorem, which states that the majority of generalised ngons can only exist for certain n. The proof, due to Kilmoyer and Solomon (1973), weaves together representation theory and graph theory.
To finish off, I will talk a little about what I've been doing here at UWA.

Friday 15 
15:00  SEMINAR  Groups and Combinatorics Seminar, ErdösKoRado sets in finite classical polar spaces

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Abstract:
ErdösKoRado sets (EKR sets) are a family of ksets of { 1, ..., n } that pairwise intersect in at least one element and were first studied by Erdös, Ko, and Rado in 1961. There are several generalizations of EKR sets. The speaker's main interest is study of EKR sets in polar spaces. These are sets of generators (maximal totally isotropic subspaces) that pairwise intersect in at least a point and were recently studied by Valentina Pepe, Leo Storme, and Frédéric Vanhove. After introducing EKR sets for sets, projective spaces, and polar spaces, some specific results using algebraic as well as geometric techniques will be presented.

Friday 22 
15:00  SEMINAR  Groups and Combinatorics Seminar, Irreducible subgroups of classical algebraic groups

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Abstract:
Let G be a group, let H be a subgroup of G and let V be an irreducible KGmodule over a field K. We say that (G,H,V) is an irreducible triple if V is an irreducible KHmodule. Classifying the irreducible triples of a group is a fundamental problem in representation theory, with a long history and several applications.
The case where G is a simple algebraic group over an algebraically closed field can be traced back to work of Dynkin in the 1950s (H connected, char(K) = 0). Through work of Seitz and Testerman in the 1980s, and more recent work of Ghandour, the problem of determining the irreducible triples (G,H,V) for simple algebraic groups has essentially been reduced to the case where G is a classical group and H is disconnected.
In this talk I will report on recent work that determines all the irreducible triples (G,H,V) when G is classical and H is a disconnected, infinite, maximal subgroup. This is an important step towards a complete classification of the irreducible triples for simple algebraic groups. I will briefly recall some of the basic results on algebraic groups and representation theory that we will need, and I will describe some of the main ideas that are used in the proofs.
This is joint work with Soumaia Ghandour, Claude Marion and Donna Testerman.

Thursday 28 
14:00  SEMINAR  Groups and Combinatorics Seminar: Bacterial genome evolution with algebra

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Andrew Francis (University of Western Sydney)
will speak on
Bacterial genome evolution with algebra
at 2pm Thursday 28th of March in Blakers Lecture Theatre
NOTE CHANGE OF DAY, TIME AND VENUE
Abstract:
The genome of a bacterial organism consists of a single circular chromosome that can undergo changes at several different levels. There is the very local level of errors that are introduced through the replication process, giving rise to changes in the nucleotide sequence (A,C,G,T); there are larger scale sequence changes occurring during the lifetime of the cell that are able to insert whole segments of foreign DNA, delete segments, or invert segments (among other things); and there are even topological changes that give rise to knotting in DNA.
Algebra might be defined as the study of ``sets with structure", and has been used over the past century to describe the symmetries of nature, most especially in areas like physics and crystallography, but it also plays a role in technological problems such a cryptography. In this talk I will describe how algebraic ideas can be used to model some bacterial evolutionary processes. In particular I will give an example in which modelling the inversion process gives rise to new algebraic questions, and show how algebraic results about the affine symmetric group can be used to calculate the ``inversion distance" between bacterial genomes. This has applications to phylogeny reconstruction.
All welcome.


April 2013

Friday 05 
15:00  SEMINAR  Groups and Combinatorics Seminar, Redeipolynomials in finite geometry

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Abstract:
L. Redei has studied in a detailed way socalled "lacunary" polynomials over finite fields. One of the applications described is to investigate the number of values the difference quotient of a polynomial over a finite field can have. This result has a direct implication in the theory of blocking sets of finite Desarguesian projective planes, and this connection is the start of the use of "Redeipolynomials" in finite geometry. We will discuss some cases to explain the principle of using Redeipolynomials finite projective spaces and some particular generalized quadrangle. Then we discuss a problem on maximal partial ovoids, that has been partially solved using Redeipolynomials, but that can be expressed in terms of transitive subsets of the group SL(2,q).

Tuesday 09 
13:00  SEMINAR  Groups and Combinatorics Seminar, The ErdosStone Theorem for finite geometries

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Abstract:
For any class of graphs, the growth function h(n) of the class is defined to be the maximum number of edges in a graph in the class on n vertices. The ErdosStone Theorem remarkably states that, for any class of graphs that is closed under taking subgraphs, the asymptotic behaviour of h(n) can (almost) be precisely determined just by the minimum chromatic number of a graph not in the class. I will present a surprising version of this theorem for finite geometries, obtained in joint work with Jim Geelen. This result is a corollary of the famous Density HalesJewett Theorem of Furstenberg and Katznelson.

Friday 12 
15:00  SEMINAR  Groups and Combinatorics Seminar, On the number of matroids

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Abstract:
Matroids are combinatorial structures that generalize graphs as well as configurations of points in projective space. They consist of a finite ground set E and a set of subsets of E called B, such that B satisfies certain axioms. We consider the problem of bounding the number m_n of matroids on a fixed ground set of size n. In 1973, Piff showed that
log log m_n < n log n + O(log log n)
In 1974, Knuth gave a lower bound of
log log m_n > n (3/2) log n + (1/2) log(2/pi)  o(1)
In the talk, I will present a recent result with Nikhil Bansal and Jorn van der Pol, that
log log m_n < n (3/2) log n + (1/2) log(2/pi) + 1 + o(1)

Friday 19 
15:00  SEMINAR  Groups and Combinatorics Seminar, Arctransitive graphs with large automorphism groups

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Abstract:
An arc in a graph is an unordered pair of adjacent vertices. A graph is called arctransitive if its automorphism group acts transitively on its set of arcs. We consider the problem of bounding the size of the automorphism group of an arctransitive graph in terms of its order. We consider the impact of the local action on this problem. (The local action is the permutation group induced by the action of the stabiliser of a vertex on its neighbourhood.) This is joint work with Primo Potocnik and Pablo Spiga.

Friday 26 
15:00  SEMINAR  Groups and Combinatorics Seminar, Locally sarc transitive regular covers of complete bipartite graphs

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Abstract:
The normal quotient method has made the study of certain families of finite graphs (for instance, sarc transitive and locally sarc transitive graphs) more approachable by dividing the problem into two parts:
(I) Study the "basic graphs," those graphs in the family that are not covers of anything but "trivial" graphs;
(II) Study the regular covers of the basic graphs.
While (I) has been studied extensively, far less work has been done toward (II).
In this talk, I will discuss how voltage graphs can be used to find regular covers of graphs where certain symmetries lift, and specifically look at the problem of determining the locally sarc transitive regular covers of complete bipartite graphs. No previous knowledge of any of these topics will be assumed.


May 2013

Friday 03 
15:00  SEMINAR  Groups and Combinatorics Seminar, On metacirculants The relationship of weak metacirculants and metacirculants

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Metacirculants were introduced by Alspach and Parsons in 1982 and have been a rich source of various topics since then, including the Hamiltonian path problem of vertextransitive graphs. A metacirculant has a vertextransitive metacyclic subgroup of automorphisms, and a longstanding curious question in the area is if the converse statement is true, namely, whether a graph with a vertextransitive metacyclic automorphism group is a metacirculant. We shall answer this question in the negative.

Friday 10 
15:00  SEMINAR  Groups and Combinatorics Seminar, Ovoids and spreads of finite polar spaces

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Abstract:
This talk is a survey of one of the driving topics in finite geometry, and the connections that ovoids and spreads have to other areas of finite geometry and permutation groups. Apart from a presentation of the history of the field and the main open problems, the speaker will give an overview of his most recent collaboration with Ferdinand Ihringer and Jan De Beule on ovoids of Hermitian polar spaces.

Friday 17 
15:00  SEMINAR  Groups and Combinatorics Seminar, Local transitivity properties of graphs and pairwise transitive designs

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Abstract:
One of the earliest triumphs in applying the finite simple group classification in algebraic graph theory was the characterization of finite distance transitive graphs. Recent work by Devillers, Giudici, Li and myself focuses on a generalisation of this class of graphs: locally $s$distance transitive graphs where ordered vertexpairs, with given first vertex and at a given distance at most $s$, are equivalent under automorphisms. One basic type of example is closely linked with the existence of very symmetrical pointline incidence structures which we call pairwise transitive designs. I will trace these developments and their links.

Friday 24 
15:00  SEMINAR  Groups and Combinatorics Seminar, Algebraic graph theory applied to configurations of polar spaces

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Abstract:
The speaker will give the second half of his talk from a fortnight ago, but with a different context. We will first go through some background on the basics of algebraic graph theory, eigenvalue techniques, and strongly regular graphs, before giving some short nonexistence proofs of ovoids and spreads in particular polar spaces.

Friday 31 
15:00  SEMINAR  Groups and Combinatorics Seminar, (k,L)complexes and graph symmetry

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Abstract:
Given an integer k and a graph L, a (k,L)complex is a polygonal complex consisting of vertices, edges and faces such that each face is a regular kgon and the graph induced by the edges and faces at each vertex is isomorphic to L. Many questions about the existence and uniquenes of (k,L)complexes and about their automorphism groups can be converted to questions about automorphisms of the graph L. I will discuss some of these ideas and report on work initiated at last year's retreat.


June 2013

Friday 07 
15:00  SEMINAR  Groups and Combinatorics Seminar, The Merino Welsh Conjecture

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Abstract:
Merino and Welsh conjectured that for any loopless bridgeless graph, the number of spanning trees is dominated either by the number of acyclic orientations of the graph, or by the number of totally cyclic orientations of the graph.
In this talk, I will discuss this conjecture and the partial results that have been previously obtained, along with a proof (which is joint work with Steve Noble) that the conjecture is true for seriesparallel graphs.

Friday 14 
15:00  SEMINAR  Groups and Combinatorics Seminar, Recognising the Symmetric and Alternating Squares

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Abstract:
The fundamental goal of Computational Group Theory is recognition: given a group somehow represented in a computer (usually by generators), determine any information about the group that you can, as quickly as possible. This is `easy' if we have enough time and memory to write the group in its entirety, but when the group is input as a generating set of matrices, this is impractical. We are explorers in an unknown land, attempting to determine the global structure of our setting from limited, local information.
The Matrix Group Recognition Project provides a recursive framework for full recognition of an arbitrary input group, by searching for a normal subgroup $N$ of $G$ and dealing with $N$ and $G/N$ separately. We present a recognition algorithm for some of the `base cases' of this recursion process, namely certain irreducible representations of a classical group, for which general methods are least effective and specialised algorithms are needed for efficiency.


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