On groups whose cogrowth series is the diagonal of a rational series

Abstract

We show that if a group contains ${\mathbb{Z}}^n\times F_m$ as a finite-index subgroup, then its cogrowth series is the diagonal of a rational function for every generating set. This answers a question of Pak and Soukup on the cogrowth of virtually abelian groups; and generalises a result by Elder, Rechnitzer, Janse van Rensburg, and Wong on the cogrowth series of the Baumslag-Solitar groups $\mathrm{BS}(N,N)$.

16 pages