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An intriguing hyperelliptic Shimura curve quotient of genus 16

Number Theory 2021-03-18 v2

Abstract

Let FF be the maximal totally real subfield of Q(ζ32)\mathbf{Q}(\zeta_{32}), the cyclotomic field of 3232nd roots of unity. Let DD be the quaternion algebra over FF ramified exactly at the unique prime above 22 and 7 of the real places of FF. Let O\mathcal{O} be a maximal order in DD, and X0D(1)X_0^D(1) the Shimura curve attached to O\mathcal{O}. Let C=X0D(1)/wDC = X_0^D(1)/\langle w_D \rangle, where wDw_D is the unique Atkin-Lehner involution on X0D(1)X_0^D(1). We show that the curve CC has several striking features. First, it is a hyperelliptic curve of genus 1616, whose hyperelliptic involution is exceptional. Second, there are 3434 Weierstrass points on CC, and exactly half of these points are CM points; they are defined over the Hilbert class field of the unique CM extension E/FE/F of class number 1717 contained in Q(ζ64)\mathbf{Q}(\zeta_{64}), the cyclotomic field of 6464th roots of unity. Third, the normal closure of the field of 22-torsion of the Jacobian of CC is the Harbater field NN, the unique Galois number field N/QN/\mathbf{Q} unramified outside 22 and \infty, with Galois group Gal(N/Q)F17=Z/17Z(Z/17Z)×\mathrm{Gal}(N/\mathbf{Q})\simeq F_{17} = \mathbf{Z}/17\mathbf{Z} \rtimes (\mathbf{Z}/17\mathbf{Z})^\times. In fact, the Jacobian Jac(X0D(1))\mathrm{Jac}(X_0^D(1)) has the remarkable property that each of its simple factors has a 22-torsion field whose normal closure is the field NN. Finally, and perhaps the most striking fact about CC, is that it is also hyperelliptic over Q\mathbf{Q}.

Keywords

Cite

@article{arxiv.1906.06772,
  title  = {An intriguing hyperelliptic Shimura curve quotient of genus 16},
  author = {Lassina Dembele},
  journal= {arXiv preprint arXiv:1906.06772},
  year   = {2021}
}

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