How long does it take to generate a group?

The diameter of a finite group $G$ with respect to a generating set $A$ is the smallest non-negative integer $n$ such that every element of $G$ can be written as a product of at most $n$ elements of $A \cup A^{-1}$. We denote this invariant by \diam_A(G). It can be interpreted as the diameter of the Cayley graph induced by $A$ on $G$ and arises, for instance, in the context of efficient communication networks. In this paper we study the diameters of a finite abelian group $G$ with respect to its various generating sets $A$. We determine the maximum possible value of \diam_A(G) and classify all generating sets for which this maximum value is attained. Also, we determine the maximum possible cardinality of $A$ subject to the condition that \diam_A(G) is "not too small". Connections with caps, sum-free sets, and quasi-perfect codes are discussed

Compressions, convex geometry and the Freiman-Bilu theorem

We note a link between combinatorial results of Bollob\'as and Leader concerning sumsets in the grid, the Brunn-Minkowski theorem and a result of Freiman and Bilu concerning the structure of sets of integers with small doubling. Our main result is the following. If eps > 0 and if A is a finite nonempty subset of a torsion-free abelian group with |A + A| <= K|A|, then A may be covered by exp(K^C) progressions of dimension [log_2 K + eps] and size at most |A|.Comment: 9 pages, slight revisions in the light of comments from the referee. To appear in Quarterly Journal of Mathematics, Oxfor

On the Bogolyubov-Ruzsa lemma

Our main result is that if A is a finite subset of an abelian group with |A+A| < K|A|, then 2A-2A contains an O(log^{O(1)} K)-dimensional coset progression M of size at least exp(-O(log^{O(1)} K))|A|.Comment: 28 pp. Corrected typos. Added appendix on model settin