English

Impossible intersections in a Weierstrass family of elliptic curves

Number Theory 2016-07-19 v2 Dynamical Systems

Abstract

Consider the Weierstrass family of elliptic curves Eλ:y2=x3+λE_{\lambda}:y^2=x^3+\lambda parametrized by nonzero λQ2\lambda\in\overline{\mathbb{Q}_2}, and let Pλ(x)=(x,x3+λ)EλP_{\lambda}(x)=(x,\sqrt{x^3+\lambda})\in E_{\lambda}. In this article, given α,βQ2\alpha,\beta\in\overline{\mathbb{Q}_2} such that αβQ\frac{\alpha}{\beta}\in\mathbb{Q}, we provide an explicit description for the set of parameters λ\lambda such that Pλ(α)P_{\lambda}(\alpha) and Pλ(β)P_{\lambda}(\beta) are simultaneously torsion for EλE_{\lambda}. In particular we prove that the aforementioned set is empty unless αβ{2,12}\frac{\alpha}{\beta}\in\{-2,-\frac{1}{2}\}. Furthermore, we show that this set is empty even when αβQ\frac{\alpha}{\beta}\notin\mathbb{Q} provided that α\alpha and β\beta have distinct 22-adic absolute values and the ramification index e(Q2(αβ)  Q2)e(\mathbb{Q}_2(\frac{\alpha}{\beta})~\vert~\mathbb{Q}_2) is coprime with 66. We also improve upon a recent result of Stoll concerning the Legendre family of elliptic curves Eλ:y2=x(x1)(xλ)E_{\lambda}:y^2=x(x-1)(x-\lambda), which itself strengthened earlier work of Masser and Zannier by establishing that provided a,ba,b have distinct reduction modulo 22, the set {λC{0,1} : (a,a(a1)(aλ)),(b,b(b1)(bλ))(Eλ)tors}\{\lambda\in\mathbb{C}\setminus\{0,1\}~:~(a,\sqrt{a(a-1)(a-\lambda)}),(b,\sqrt{b(b-1)(b-\lambda)})\in (E_{\lambda})_{tors}\} is empty.

Keywords

Cite

@article{arxiv.1507.07047,
  title  = {Impossible intersections in a Weierstrass family of elliptic curves},
  author = {Niki Myrto Mavraki},
  journal= {arXiv preprint arXiv:1507.07047},
  year   = {2016}
}

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16 pages