1.3. Gromov’s theorem 21
Let us say that a finite group G is a group of polynomial growth if one
has |B(1,R)|
CRd
for all R 1 and some constants C, d 0.
Exercise 1.3.2. Show that the notion of a group of polynomial growth (as
well as the rate d of growth) does not depend on the choice of generators
S; thus if S is another set of generators for G, show that G has polynomial
growth with respect to S with rate d if and only if it has polynomial growth
with respect to S with rate d.
Exercise 1.3.3. Let G be a finitely generated group, and let G be a finite
index subgroup of G.
(i) Show that G is also finitely generated. (Hint: Let S be a symmetric
set of generators for G containing the identity, and locate a finite
integer n such that Sn+1G = SnG . Then show that the set S :=
G
S2n+1
is such that
Sn+1

SnS
. Conclude that
Sn
meets
every coset of S (or equivalently that G =
Sn
S ), and use this
to show that S generates G .)
(ii) Show that G has polynomial growth if and only if G has polynomial
growth.
(iii) More generally, show that any finitely generated subgroup of a
group of polynomial growth also has polynomial growth. Conclude
in particular that a group of polynomial growth cannot contain the
free group on two generators.
From Exercise 1.2.2 we see that the discrete Heisenberg group (1.5) is of
polynomial growth. It is in fact not difficult to show that, more generally,
any nilpotent finitely generated group is of polynomial growth. By Exercise
1.3.3, this implies that any virtually nilpotent finitely generated group is of
polynomial growth.
Gromov’s theorem asserts the converse statement:
Theorem 1.3.1 (Gromov’s theorem [Gr1981]). Let G be an object with
the following properties:
(1) (Group-like object) G is a finitely generated group.
(2) (Weak regularity) G is of polynomial growth.
Then there exists a subgroup G of G such that
(i) (G is close to G) The index |G/G | is finite.
(ii) (Nilpotent type structure) G is nilpotent.
More succinctly: A finitely generated group is of polynomial growth if
and only if it is virtually nilpotent.
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