The ranks of twists of an elliptic curve in characteristic $3$

Abstract

Starting from the elliptic curve $E:y^2=x^3-x$ over ${\mathbb{F}}_9$, a curve $\mathcal{X}$ over ${\mathbb{F}}_{3^{2n}}$ and a cyclic cover of $\mathcal{X}$ of degree $m\in \{2,3,4,6\}$, we construct the corresponding $m$-twists over the function field ${\mathbb{F}}_{3^{2n}}(\mathcal{X})$. We also obtain the Mordell-Weil rank of these twists in terms of the Zeta functions of $\mathcal{X}$ and of suitable Kummer and Artin-Schreier extensions of it. Finally, we also describe the fibers of the elliptic fibration associated to such twists in the case $\mathcal{X}={\mathbb{P}}^1$.