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 (so the term approximate subgroup may be more correct). For example, it is required that the set of products of elements in the subset be not much bigger than the subset itself (while for a subgroup it is required that they be equal). The notion was introduced in the 2010s but can be traced to older sources in additive combinatorics.
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:
- It is symmetric, that is if then ;
- There exists a subset of cardinality such that .
It is immediately verified that a 1-approximate subgroup is the same thing as a genuine subgroup. Of course this definition is only interesting when is small compared to (in particular, any subset is a -approximate subgroup). In applications it is often used with 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.
Classification of approximate subgroups
- For any there are such that for any -approximate subgroup there exists a generalised arithmetic progression generated by at most integers and containing at least elements, such that .
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 (the culmination of an effort started a few years earlier by various other people) is a vast generalisation of this result. In a very general form its statement is the following:
- Let ; there exists such that the following holds. Let be a group and a -approximate subgroup in . There exists subgroups with finite and nilpotent such that , the subgroup generated by contains , and with .
The statement also gives some information on the characteristics (rank and step) of the nilpotent group .
- Let . For any there is a constant such that for any finite field , any simple subgroup and any -approximate subgroup then either is contained in a proper subgroup of , or , or .
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 (besides genuine subgroups) in finite simple linear groups (they are either "trivial", that is very small, or "not proper", that is almost equal to the whole group).
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 (of polynomial growth) and other groups; that is, there exists a (superpolynomial) function such that any group with growth function bounded by a multiple of is virtually nilpotent.
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- Breuillard, Green & Tao 2012, Theorem 2.1.
- Breuillard, Green & Tao 2012, Theorem 1.6.
- Breuillard 2015, Theorem 4.8.
- Breuillard, Green & Tao 2012, Theorem 1.11.
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- Breuillard, Emmanuel (2014). "Expander graphs, property (τ) and approximate groups". In Bestvina, Mladen; Sageev, Michah; Vogtmann, Karen. Geometric group theory (PDF). IAS/Park City mathematics series. 21. American Math. Soc. pp. 325–378.
- Breuillard, Emmanuel; Tao, Terence; Green, Ben (2012). "The structure of approximate groups". Publ. Math. IHES. 116: 115–221. arXiv: . doi:10.1007/s10240-012-0043-9.
- Green, Ben (May 2012). "What is... an approximate group?" (PDF). Notices of the AMS. 59 (5).