Approximate group


In mathematics, an approximate group is a subset of a group which behaves like a subgroup "up to a constant error", in a precise quantitative sense. For example, it is required that the set of products of elements in the subset be not much bigger than the subset itself. The notion was introduced in the 2010s but can be traced to older sources in additive combinatorics.

Formal definition

Let be a group and ; for two subsets we denote by the set of all products. A non-empty subset is a -approximate subgroup of if:
  1. It is symmetric, that is if then ;
  2. There exists a subset of cardinality such that.
It is immediately verified that a finite 1-approximate subgroup is the same thing as a genuine subgroup. Of course this definition is only interesting when is small compared to . In applications it is often used with K being fixed and going to infinity.
Examples of approximate subgroups which are not groups are given by symmetric intervals and more generally arithmetic progressions in the integers. Indeed, for all the subset is a 2-approximate subgroup: the set is contained in the union of the two translates and of. A generalised arithmetic progression in is a subset in of the form, and it is a -approximate subgroup.
A more general example is given by balls in the word metric in finitely generated nilpotent groups.

Classification of approximate subgroups

Approximate subgroups of the integer group were completely classified by Imre Z. Ruzsa and Freiman. The result is stated as follows:
The constants can be estimated sharply. In particular is contained in at most translates of : this means that approximate subgroups of are "almost" generalised arithmetic progressions.
The work of Breuillard-Green-Tao is a vast generalisation of this result. In a very general form its statement is the following:
The statement also gives some information on the characteristics of the nilpotent group.
In the case where is a finite matrix group the results can be made more precise, for instance:
The theorem applies for example to ; the point is that the constant does not depend on the cardinality of the field. In some sense this says that there are no interesting approximate subgroups in finite simple linear groups.

Applications

The Breuillard-Green-Tao theorem on classification of approximate groups can be used to give a new proof of Gromov's theorem on groups of polynomial growth. The result obtained is actually a bit stronger since it establishes that there exists a "growth gap" between virtually nilpotent groups and other groups; that is, there exists a function such that any group with growth function bounded by a multiple of is virtually nilpotent.
Other applications are to the construction of expander graphs from the Cayley graphs of finite simple groups, and to the related topic of superstrong approximation.