# Constructing hyperelliptic curves with surjective Galois representations

## TL;DR

This paper constructs explicit equations for hyperelliptic curves over Q with surjective Galois representations mod l, advancing understanding of Galois actions on Jacobians and providing a systematic framework for primitivity of symplectic representations.

## Contribution

It provides explicit equations for hyperelliptic curves with surjective Galois representations for all odd primes l, based on prime sum conditions and inertia group analysis.

## Key findings

- Explicit equations for hyperelliptic curves with surjective Galois images
- Framework for analyzing primitivity of symplectic Galois representations
- Conditions on n and prime sums ensure surjectivity for all primes l

## Abstract

In this paper we show how to explicitly write down equations of hyperelliptic curves over Q such that for all odd primes l the image of the mod l Galois representation is the general symplectic group. The proof relies on understanding the action of inertia groups on the l-torsion of the Jacobian, including at primes where the Jacobian has non-semistable reduction. We also give a framework for systematically dealing with primitivity of symplectic mod l Galois representations. The main result of the paper is the following. Suppose n=2g+2 is an even integer that can be written as a sum of two primes in two different ways, with none of the primes being the largest primes less than n (this hypothesis appears to hold for all g different from 0,1,2,3,4,5,7 and 13). Then there is an explicit integer N and an explicit monic polynomial $f_0(x)\in \mathbb{Z}[x]$ of degree n, such that the Jacobian $J$ of every curve of the form $y^2=f(x)$ has $Gal(\mathbb{Q}(J[l])/\mathbb{Q})\cong GSp_{2g}(\mathbb{F}_l)$ for all odd primes l and $Gal(\mathbb{Q}(J[2])/\mathbb{Q})\cong S_{2g+2}$, whenever $f(x)\in\mathbb{Z}[x]$ is monic with $f(x)\equiv f_0(x) \bmod{N}$ and with no roots of multiplicity greater than $2$ in $\overline{\mathbb{F}}_p$ for any p not dividing N.