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.
Previous Page Next Page