Torsion points of small order on cyclic covers of $\mathbb P^1$. II
Abstract
Let be an integer, a perfect field such that does not divide , an integer prime to , a degree monic polynomial without repeated roots, and a smooth projective model of the affine curve . Let be the Jacobian of the -curve . We identify with its canonical image in (such that the infinite point of goes to the zero of the group law on ). We say that an integer is -reachable over if there exists a polynomial as above such that contains a torsion point of order . Earlier we proved that if is -reachable, then either or (in addition, both and are -reachable). In the present paper we prove the following. If and if is -reachable over , then either or . If either or in infinite and , then is -reachable if and only if . If , then is -reachable if and only if . If (the hyperelliptic case) and , then is -reachable if . (The case when was done earlier by E.V. Flynn.)
Cite
@article{arxiv.2510.16912,
title = {Torsion points of small order on cyclic covers of $\mathbb P^1$. II},
author = {Boris Bekker and Yuri G. Zarhin},
journal= {arXiv preprint arXiv:2510.16912},
year = {2025}
}
Comments
33 pages. We added results about torsion points over arbitrary infinite perfect fields, including the most interesting case of the field of rational numbers