English

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

Number Theory 2026-01-21 v1 Algebraic Geometry

Abstract

Let d>1d>1 be an integer and K0K_0 a perfect field such that char(K0)char(K_0) does not divide dd. Let n>dn>d be an integer that is prime to dd. Let f(x)K0[x]f(x)\in K_0[x] be a degree nn monic polynomial without repeated roots, and Cf,d\mathcal{C}_{f,d} a smooth projective model of the affine curve yd=f(x)y^d=f(x). Let J(Cf,d)J(\mathcal{C}_{f,d}) be the Jacobian of the K0K_0-curve Cf,d\mathcal{C}_{f,d} . As usual, we identify Cf,d\mathcal{C}_{f,d} with its canonical image in J(Cf,d)J(\mathcal{C}_{f,d}) (such that the only ``infinite point'' of Cf,d\mathcal{C}_{f,d} goes to the zero of the group law on J(Cf,d)J(\mathcal{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)\mathcal{C}_{f,d}(K_0) contains a torsion point of order mm. Let us put 0:=[(n+d)/d], m0:=0d\ell_0:=[(n+d)/d], \ m_0:=\ell_0 d. Earlier we proved that if mm is (n,d)(n,d)-reachable, then either m=dm=d or m=nm = n or mm0m \ge m_0 (in addition, both dd and nn are (n,d)(n,d)-reachable over every K0K_0). We also proved that if m0m_0 is (n,d)(n,d)-reachable over some K0K_0 then nm0+00n-m_0+\ell_0\ge 0. In the present paper we discuss the (n,d)(n,d)-reachability of m0m_0 when nm0+0=0n-m_0+\ell_0=0 or 11.

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Cite

@article{arxiv.2601.12643,
  title  = {Torsion points of small order on cyclic covers of $\mathbb{P}^1$. III},
  author = {Boris M. Bekker and Yuri G. Zarhin},
  journal= {arXiv preprint arXiv:2601.12643},
  year   = {2026}
}

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24 pages