1.2. Approximate groups 15
Figure 1. A schematic description of the steps needed to establish the
Gleason-Yamabe theorem. The annotation O, Q on an arrow indicates
that one has to pass to an open subgroup, and then quotient out a
compact normal subgroup, in order to obtain the additional structure
at the end of the arrow.
There are several ways to quantify what it means for a set A to be
“almost” closed under addition or multiplication. Here are some common
formulations of this idea (phrased in multiplicative notation, for the sake of
concreteness):
(1) (Statistical multiplicative structure) For a “large” proportion of
pairs (a, b) A × A, the product ab also lies in A.
(2) (Small product set) The “size” of the product set A · A := {ab :
a, b A} is “comparable” to the “size” of the original set A. (For
technical reasons, one sometimes uses the triple product A·A·A :=
{abc : a, b, c A} instead of the double product.)
(3) (Covering property) The product set A · A can be covered by a
“bounded” number of (left or right) translates of the original set
A.
Of course, to make these notions precise one would have to precisely quan-
tify the various terms in quotes. Fortunately, the basic theory of additive
combinatorics (and multiplicative combinatorics) can be used to show that
all these different notions of additive or multiplicative structure are “es-
sentially” equivalent; see [TaVu2006, Chapter 2] or [Ta2008b] for more
discussion.
Previous Page Next Page