Densities of eigenspaces of Frobenius and distributions of R-modules

Abstract

For any polynomial $p\left(x\right)$ over ${\mathbb{F}}_l$ we determine the asymptotic density of hyperelliptic curves over ${\mathbb{F}}_q$ of genus $g$ for which $p\left(x\right)$ divides the characteristic polynomial of Frobenius acting on the $l$-torsion of the Jacobian, and give an explicit formula for this density. We prove this result as a consequence of more general density theorems for quotients of Tate modules of such curves, viewed as modules over the Frobenius. The proof involves the study of measures on $R$-modules over arbitrary rings $R$ which are finite ${\mathbb{Z}}_l$-algebras. In particular we prove a result on the convergence of sequences of such measures, which can be applied to the moments computed in recent work of Lipnowski-Tsimerman to obtain the above results. We also extend the random model for finite $R$-modules proposed in that work to such rings $R$, and prove several of its properties. Notably the measure obtained is in general not inversely proportional to the size of the automorphism group.