Exponential growth rates of free and amalgamated products

TL;DR

This paper investigates the exponential growth rates of free and amalgamated products, establishing new bounds and explicit growth rate values, thereby answering open questions in group theory.

Contribution

It proves a gap in exponential growth rates for free products and determines explicit growth rates for certain amalgamated products, advancing understanding of growth behaviors.

Findings

A gap between  and + in growth rates of free products.

Explicit growth rate for    is the root of z^3 - z - 1.

Lower exponential growth rates than  are achievable in certain amalgamated products.

Abstract

We prove that there is a gap between $\sqrt{2}$ and $(1+\sqrt{5})/2$ for the exponential growth rate of free products $G=A*B$ not isomorphic to the infinite dihedral group. For amalgamated products $G=A*_C B$ with $([A:C]-1)([B:C]-1)\geq2$, we show that lower exponential growth rate than $\sqrt{2}$ can be achieved by proving that the exponential growth rate of the amalgamated product $\mathrm{PGL}(2,\mathbb{Z})\cong (C_2\times C_2) *_{C_2} D_6$ is equal to the unique positive root of the polynomial $z^3-z-1$. This answers two questions by Avinoam Mann [The growth of free products, Journal of Algebra 326, no. 1 (2011) 208--217].