English

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

Algebraic Geometry 2025-12-18 v2 Number Theory

Abstract

Let d2d\geq 2 be an integer, K0K_0 a perfect field such that char(K0)char(K_0) does not divide dd, n>dn > d an integer prime to dd, f(x)K0[x]f(x)\in K_0[x] a degree nn monic polynomial without repeated roots, and Cf,dC_{f,d} a smooth projective model of the affine curve yd=f(x)y^d=f(x). Let J(Cf,d)J(C_{f,d}) be the Jacobian of the K0K_0-curve Cf,dC_{f,d} . We identify Cf,dC_{f,d} with its canonical image in J(Cf,d)J(C_{f,d}) (such that the infinite point of Cf,dC_{f,d} goes to the zero of the group law on J(Cf,d)J(C_{f,d})). We say that an integer m>1m>1 is (n,d)(n,d)-reachable over K0K_0 if there exists a polynomial f(x)f(x) as above such that Cf,d(K0)C_{f,d}(K_0) contains a torsion point of order mm. Earlier we proved that if mm is (n,d)(n,d)-reachable, then either m=dm=d or mnm \geq n (in addition, both dd and nn are (n,d)(n,d)-reachable). In the present paper we prove the following. If n<m<2nn<m<2n and if mm is (n,d)(n,d)-reachable over K0K_0, then either dmd|m or mnmoddm \equiv n \bmod d. If either char(K0)=0char(K_0)=0 or K0K_0 in infinite and char(K0)>nchar(K_0)>n, then d[(n+d)/d]d\cdot [(n+d)/d] is (n,d)(n,d)-reachable if and only if n(d1)[(n+d)/d]0n-(d-1)\cdot [(n+d)/d]\ge 0. If char(K0)=0char(K_0)=0, then n+dn+d is (n,d)(n,d)-reachable if and only if d22d<nd^2-2d<n. If d=2d=2 (the hyperelliptic case) and char(K0)=0char(K_0)=0, then mm is (n,d)(n,d)-reachable if n+1m2n+1n+1 \le m \le 2n+1. (The case when nm3(n1)/2n \le m \le 3(n-1)/2 was done earlier by E.V. Flynn.)

Keywords

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