“…Freiman very precisely in the additive group of the integers (see [4], [5], [6], [7]) and by many other authors in general abelian groups, starting with M. Kneser [16] (see, for example, [15], [17], [1], [23], [14]). More recently, small doubling problems in non-necessarily abelian groups have been also studied, see [13], [24] and [3] for recent surveys on these problems and [19] and [26] for two important books on the subject. It is easy to prove that if S is a finite subset of Z, then 2|S| − 1 ≤ |2S| ≤ |S|(|S| + 1) 2 .…”