Average Numbers of Homomorphisms to Random Modules over Free Group Algebras

Let $F$ be a finitely generated free group, and let $L$ be a finitely presented ${\mathbb{F}}_q[F]$-module. We study the average number ${\mathrm{\Lambda }}_L(n)$ of ${\mathbb{F}}_q[F]$-module homomorphisms from $L$ to an ${\mathbb{F}}_q[F]$-module of dimension $n$ over ${\mathbb{F}}_q$. We show that, for all sufficiently large $n$, the quantity ${\mathrm{\Lambda }}_L(n)$ is given by a rational function of $q^n$ and satisfies $${\mathrm{\Lambda }}_L(n)=q^{\chi (L)n}+\sum _{N\in A(L)}{q}^{\chi (L\mathrm{/}N)n}(1+O(q^{-n})),$$ where $\chi (L)$ denotes the Euler characteristic of $L$, and $A(L)$ is the set of nonzero ${\mathbb{F}}_q[F]$-submodules $N$ of $L$ that have no nonzero free quotients. Our proof is based on a theory of partial modules that may be of independent interest and have further applications.