The Mathlib/Geometry/Group/Growth subfolder contains mathematical results concerning the growth of finitely generated groups.
For a finitely generated group $G = \langle S\rangle$ with a finite symmetric generating set $S$, the growth is defined by the function $n \mapsto |S^n|$, where $S^n$ is the set of elements resulting from the pointwise multiplication of $n$ copies of $S$. The growth rate is considered invariant (up to scaling) regardless of the choice of the symmetric generating set $S$.
Currently covered topics include:
- Proofs that the growth of a group is at least linear.
- Results relating the growth of a group $G$ to the growth of a normal subgroup $H \le G$ and the growth of the quotient group $G / H$ (specifically that the growth of $G$ is roughly the product of the growth of $H$ and $G/H$).