Growth rate (group theory)
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In group theory, the growth rate of a group with respect to a symmetric generating set describes the size of balls in the group. Every element in the group can be written as a product of generators, and the growth rate counts the number of elements that can be written as a product of length n.
Let us consider the subset of all elements of G which can be presented by such a word of length ≤ n
More geometrically, is the set of vertices in the Cayley graph with respect to T which are within distance n of the identity.
Given two nondecreasing positive functions a and b one can say that they are equivalent () if there is a constant C such that
for example if .
Then the growth rate of the group G can be defined as the corresponding equivalence class of the function
where denotes the number of elements in the set . Although the function depends on the set of generators T its rate of growth does not (see below) and therefore the rate of growth gives an invariant of a group.
The word metric d and therefore sets depend on the generating set T. However, any two such metrics are bilipschitz equivalent in the following sense: for finite symmetric generating sets E, F, there is a positive constant C such that
As an immediate corollary of this inequality we get that the growth rate does not depend on the choice of generating set.
Polynomial and exponential growth
for some we say that G has a polynomial growth rate. The infimum of such k's is called the order of polynomial growth. According to Gromov's theorem, a group of polynomial growth is a virtually nilpotent group, i.e. it has a nilpotent subgroup of finite index. In particular, the order of polynomial growth has to be a natural number and in fact .
- A free group with a finite rank k > 1 has an exponential growth rate.
- A finite group has constant growth – polynomial growth of order 0 – and includes fundamental groups of manifolds whose universal cover is compact.
- If M is a closed negatively curved Riemannian manifold then its fundamental group has exponential growth rate. Milnor proved this using the fact that the word metric on is quasi-isometric to the universal cover of M.
- Zd has a polynomial growth rate of order d.
- The discrete Heisenberg group H3 has a polynomial growth rate of order 4. This fact is a special case of the general theorem of Bass and Guivarch that is discussed in the article on Gromov's theorem.
- The lamplighter group has an exponential growth.
- The existence of groups with intermediate growth, i.e. subexponential but not polynomial was open for many years. It was asked by Milnor in 1968 and was finally answered in the positive by Grigorchuk in 1984. There are still open questions in this area and a complete picture of which orders of growth are possible and which are not is missing.
- The triangle groups include 3 finite groups (the spherical ones, corresponding to sphere), 3 groups of quadratic growth (the Euclidean ones, corresponding to Euclidean plane), and infinitely many groups of exponential growth (the hyperbolic ones, corresponding to the hyperbolic plane).
- J. Milnor, A note on curvature and fundamental group, J. Differential Geometry 2 (1968), 1–7.
- R. I. Grigorchuk, Degrees of growth of finitely generated groups and the theory of invariant means., Izv. Akad. Nauk SSSR Ser. Mat. 48:5 (1984), 939–985 (Russian).