Dead ends and rationality of complete growth series

Abstract

The complete growth series of a finitely generated group is given by ${\sum }_{n\ge 0}{A}_n{s}^n$, where $A_n$ is the sum of elements of length $n$ in the group semiring. We study the $\mathbb{N}G$-rationality and $\mathbb{N}G$-algebraicity of such series. We show that having dead ends of arbitrarily large depths is an obstruction to $\mathbb{N}G$-rationality. In the case of the $3$-dimensional Heisenberg group $H_3(\mathbb{Z})$, we prove that the complete series is not $\mathbb{N}G$-algebraic for any generating set. Dead ends are also used to show that complete growth series of higher Heisenberg groups are not $\mathbb{N}G$-rational for specific generating sets. Using a more general version of this obstruction, we prove that complete growth series of some lamplighter groups are not $\mathbb{N}G$-rational either.

20 pages, 9 figures. v2: Improved presentation for Section 3

References in corpus (3)

• Cone types and geodesic languages for lamplighter groups and Thompson's group F
• Dead ends on wreath products and lamplighter groups
• On the speed of convergence to the asymptotic cone for non-singular nilpotent groups