Irrationality of the conjugacy growth on Heisenberg groups

Abstract

We demonstrate that the conjugacy growth series for the Heisenberg groups $H_n(\mathbb{Z})$ is never algebraic, irrespective of the generating set. This makes progress towards the conjecture that all finitely presented groups that are not virtually abelian have irrational conjugacy growth series. Contrary to the group $H_3(\mathbb{Z})$, where the conjugacy growth is $n^2\log(n)$, the conjugacy growth of the other Heisenberg groups is polynomial, making these groups the first known nilpotent groups where the irrationality does not follow from the asymptotic growth.