Torsion points of small order on cyclic covers of ${\mathbb{P}}^1$. II

Abstract

Let $d\ge 2$ be an integer, $K_0$ a perfect field such that $char(K_0)$ does not divide $d$, $n>d$ an integer prime to $d$, $f(x)\in K_0[x]$ a degree $n$ monic polynomial without repeated roots, and $C_{f,d}$ a smooth projective model of the affine curve $y^d=f(x)$. Let $J(C_{f,d})$ be the Jacobian of the $K_0$-curve $C_{f,d}$. We identify $C_{f,d}$ with its canonical image in $J(C_{f,d})$ (such that the infinite point of $C_{f,d}$ goes to the zero of the group law on $J(C_{f,d})$). We say that an integer $m>1$ is $(n,d)$-reachable over $K_0$ if there exists a polynomial $f(x)$ as above such that $C_{f,d}(K_0)$ contains a torsion point of order $m$. Earlier we proved that if $m$ is $(n,d)$-reachable, then either $m=d$ or $m\ge n$ (in addition, both $d$ and $n$ are $(n,d)$-reachable). In the present paper we prove the following. If $n<m<2n$ and if $m$ is $(n,d)$-reachable over $K_0$, then either $d|m$ or $m\equiv n\mathrm{mod}d$. If either $char(K_0)=0$ or $K_0$ in infinite and $char(K_0)>n$, then $d\cdot [(n+d)/d]$ is $(n,d)$-reachable if and only if $n-(d-1)\cdot [(n+d)/d]\ge 0$. If $char(K_0)=0$, then $n+d$ is $(n,d)$-reachable if and only if $d^2-2d<n$. If $d=2$ (the hyperelliptic case) and $char(K_0)=0$, then $m$ is $(n,d)$-reachable if $n+1\le m\le 2n+1$. (The case when $n\le m\le 3(n-1)/2$ was done earlier by E.V. Flynn.)

33 pages. We added results about torsion points over arbitrary infinite perfect fields, including the most interesting case of the field of rational numbers