English

Noncommutative sets of small doubling

Combinatorics 2012-04-04 v2

Abstract

A corollary of Kneser's theorem, one sees that any finite non-empty subset AA of an abelian group G=(G,+)G = (G,+) with A+A(2\eps)A|A + A| \leq (2-\eps) |A| can be covered by at most 2\eps1\frac{2}{\eps}-1 translates of a finite group HH of cardinality at most (2\eps)A(2-\eps)|A|. Using some arguments of Hamidoune, we establish an analogue in the noncommutative setting. Namely, if AA is a finite non-empty subset of a nonabelian group G=(G,)G = (G,\cdot) such that AA(2\eps)A|A \cdot A| \leq (2-\eps) |A|, then AA is either contained in a right-coset of a finite group HH of cardinality at most 2\epsA\frac{2}{\eps}|A|, or can be covered by at most 2\eps1\frac{2}{\eps}-1 right-cosets of a finite group HH of cardinality at most A|A|. We also note some connections with some recent work of Sanders and of Petridis.

Keywords

Cite

@article{arxiv.1106.2267,
  title  = {Noncommutative sets of small doubling},
  author = {Terence Tao},
  journal= {arXiv preprint arXiv:1106.2267},
  year   = {2012}
}

Comments

8 pages, no figures. To appear, European Journal of Combinatorics. This is the final version, incorporating the referee corrections

R2 v1 2026-06-21T18:21:00.566Z