English

Curve equations from expansions of 1-forms at a nonrational point

Number Theory 2025-06-18 v1

Abstract

We exhibit an algorithm to compute equations of an algebraic curve over a computable characteristic 0 field from the power series expansions of its regular 1-forms at a nonrational point of the curve, extending a 2005 algorithm of Baker, Gonz\'alez-Jim\'enez, Gonz\'alez, and Poonen for expansions at a rational point. If the curve is hyperelliptic, the equations present it as an explicit double cover of a smooth plane conic, or as a double cover of the projective line when possible. If the curve is nonhyperelliptic, the equations cut out the canonical model. The algorithm has been used to compute equations over Q\mathbb{Q} for many hyperelliptic modular curves without a rational cusp in the L-functions and Modular Forms Database.

Keywords

Cite

@article{arxiv.2506.14026,
  title  = {Curve equations from expansions of 1-forms at a nonrational point},
  author = {Raymond van Bommel and Edgar Costa and Bjorn Poonen and Padmavathi Srinivasan},
  journal= {arXiv preprint arXiv:2506.14026},
  year   = {2025}
}

Comments

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