Growth of Approximate Groups in Hyperbolic Groups · arXivDesk
2606.13632Jun 11, 2026In this new version, we added a combinatorial proof for a sphere expansion property in hyperbolic groups, as well as adding references
We prove a growth dichotomy for infinite approximate groups, and more generally approximate semigroups, in hyperbolic groups. If G is a finitely generated hyperbolic group and A⊆G is infinite with A2⊆AX for some finite X⊆G
Nearby in the stack
, then either
⟨A⟩
is virtually cyclic, or
A
has positive exponential growth in the ambient word metric. We also introduce a product-growth criterion for the existence of growth rates of approximate semigroups. The criterion applies to hyperbolic groups: if
G
is hyperbolic with finite generating set
S
, then there is a constant
cG,S>0
such that
∣UV∣≥cG,Sn+k+1∣U∣∣V∣,U⊆Bn,V⊆Bk.
The linear loss is optimal in order whenever
G
contains an element of infinite order. In the free group with its standard generating set one may take