Normal zeta function of a group

Definition

Let $G$ be a group. The normal zeta function of $G$ is defined as:

$\zeta _{G}(s)=\sum _{n=1}^{\infty }a_{n}(G)n^{-s}$

where $a_{n}(G)$ denotes the number of normal subgroups of $G$ of index $n$. Equivalently, it is:

$\sum _{H{\underline {\triangleleft }}_{f}G}[G:H]^{-s}$

summing up over all normal subgroups of finite index in $G$.

The coefficients $a_{n}(G)$ are all finite when the group $G$ is finitely generated. This follows from finitely generated implies finitely many homomorphisms to any finite group (see also group with finitely many homomorphisms to any finite group).

Related notions

• Zeta function of a group