Expanders and growth in groups
These are the web pages for a set of seminars to be given at the University of Western Australia in 2010. The seminars constitute an introduction to the area of “growth in groups”, and the connection between growth and “expander graphs”.
The research group at UWA is very strong in group theory and in finite geometry, hence I will emphasise these aspects of the subject. I will also assume familiarity with results from these areas.
 Recommended background reading can be found here.
 Lectures (I will add lecture notes after the lecture is given):
 Sumproduct (12 noon, Tues 4 May 2010; notes now available) We introduce the idea of growth in groups, before focussing on the abelian setting. We take a first look at the sumproduct principle, with a brief foray into the connection between sumproduct results and incidence theorems.
We then focus on Helfgott’s restatement of the sumproduct principle in terms of groups acting on groups.

Growth in groups of Lie type (11am, Fri 7 May 2010; notes now available) Since Helfgott first proved that “generating sets grow” in SL_2(p) and SL_3(p), our understanding of how to prove such results has developed a great deal. It is now possible to prove that generating sets grow in any finite group of Lie type; what is more the most recent proofs are very direct – they have no recourse to the incidence theorems of Helfgott’s original approach. We give an overview of this new approach, which has come to be known as a “pivotting argument”. There are five parts to this approach, and we outline how these fit together.

Escape (11am, Fri 14 May 2010); notes now available) The principle of “escape from subvarieties” is the first step in proving growth in groups of Lie type. We give a proof of this result, and its most important application (for us) – the construction of regular semisimple elements. We then examine other related ideas from algebraic geometry, in particular the idea of nonsingularity.

Growth in soluble subgroups of GL_r(p) (11am, Wed 21 Jul 2010; notes will not be written up for this – a preprint will appear on the arXiv in due course.) We show how to reduce the study of exponential growth in soluble subgroups of GL_r(p) to the nilpotent setting. We make use of ideas based on the sumproduct phenomenon, as well as some machinery from linear algebraic groups. We will not assume any background from these areas. This lecture is based on new results of the lecturer and Helfgott.

An introduction to expanders (11am, Fri 23 Jul 2010; notes now available) This is a background lecture preparing the way for the final lecture, where we connect results on growth in simple groups to the explicit construction of families of expanders. In this lecture we will define what we mean by a family of expanders, stating (and sometimes even proving!!) background results that will be important later. We will also try to explain why expanders are of such interest to so many different groups of people.

Using growth results to explicitly construct expanders (1pm, Tue 27 Jul 2010; notes now available) We outline the method of Bourgain and Gamburd. They were the first to use results concerning growth in simple groups to explicitly construct expander graphs. Let S be a set in SL_2(Z) and define S_p to be the natural projection of S modulo p. Now let G_p be the Cayley graph of SL_2(p) with respect to the set S_p. Bourgain and Gamburd give precise results as to when the set of graphs {G_p : p a prime} forms a family of expanders. They make crucial use of the result of Helfgott (encountered in Seminar 2) which states that “all generating sets in SL_2(p) grow”.