Densities of eigenspaces of Frobenius and distributions of R-modules

TL;DR

This paper calculates the asymptotic density of hyperelliptic curves over finite fields with specific Frobenius characteristic polynomial divisibility, extending random models for R-modules and analyzing measures on modules over finite rings.

Contribution

It provides explicit density formulas for Frobenius eigenspaces and extends the random R-module model to finite rings, with convergence results for measures on modules.

Findings

Derived explicit density formulas for polynomials dividing Frobenius characteristic polynomials.

Extended the random R-module model to finite rings and analyzed its properties.

Proved convergence of measures on R-modules, applicable to moments in recent research.

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.