Exponential growth rates of free and 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 $G=A\ast B$ not isomorphic to the infinite dihedral group. For amalgamated products $G=A{\ast }_{C}B$ with $([A:C]-1)([B:C]-1)\ge 2$, we show that lower exponential growth rate than $\sqrt{2}$ can be achieved by proving that the exponential growth rate of the amalgamated product $\mathrm{PGL}(2,\mathbb{Z})\cong (C_2\times C_2){\ast }_{C_2}{D}_6$ 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].

17 pages, 7 figures