Curves in characteristic $2$ with non-trivial $2$-torsion

Abstract

Cais, Ellenberg and Zureick-Brown recently observed that over finite fields of characteristic two, all sufficiently general smooth plane projective curves of a given odd degree admit a non-trivial rational 2-torsion point on their Jacobian. We extend their observation to curves given by Laurent polynomials with a fixed Newton polygon, provided that the polygon satisfies a certain combinatorial property. We also show that in each of these cases, the sufficiently general condition is implied by being ordinary. Our treatment includes many classical families, such as hyperelliptic curves of odd genus and C a,b curves. In the hyperelliptic case, we provide alternative proofs using an explicit description of the 2-torsion subgroup.

2‐Selmer parity for hyperelliptic curves in quadratic extensions

“…One readily checks that the equation defining 𝐶 is smooth, and hence defines a hyperelliptic curve over F2 of genus g. Let 𝐽 be the Jacobian of 𝐶. As in the proof of [14,Theorem 23], one sees that dim 𝐽( F2 ) [2]…”

“…Let $$J$$ be the Jacobian of $$C$$. As in the proof of [14, Theorem 23], one sees that $$\dim J(\bar{\mathbb {F}}_2)[2]=g$$, and hence $$J$$ is ordinary.$$\Box$$…”