Explicit Galois obstruction and descent for hyperelliptic curves with tamely cyclic reduced automorphism group.

*(English)*Zbl 1343.14047Let \(K\) be the algebraic closure of a perfect field with \(\mathrm{char}(K)\neq 2\), and let \(F\) be a subfield of \(K\). Let \(X\) be a variety defined over \(F\), and the field of moduli \(k\) of \(X\) is the intersection of all subfields \(L\) of \(K\) such that there is a variety \(Y\) defined over \(L\) and \(X_{K}\cong Y_K\). The field of moduli \(k\) of \(X\) has the property that \(X_K \cong X_K^\sigma\) for \(\sigma\in\mathrm{Aut}(K)\) if and only if \(\sigma\in \mathrm{Gal}(K/k)\). In [Am. J. Math. 78, 509–524 (1956; Zbl 0072.16001)], A. Weil asked if there is a model of \(X\) over the field of moduli \(k\). Such a model, if exists, is called a descent of \(X\), and we say, \(X\) has a descent for the extension \(K/k\). Weil showed that if \(\mathrm{Aut}(X_K)\) is trivial, then a descent of \(X\) exists.

If \(X\) is a hyperelliptic curve, the presence of the hyperelliptic involution \(\iota\) makes \(\mathrm{Aut}(X_K)\) nontrivial. If we further require the existence of a model in the form of a hyperelliptic equation \(y^2=p(x)\) over \(k\), we call such a model a hyperelliptic descent of \(X\). Let \(G=\mathrm{Aut}(X_K)\), and \(\overline{G}=G/\langle \iota \rangle\). In [B. Huggins, Math. Res. Lett. 14, No. 2, 249–262 (2007; Zbl 1126.14036)], it was shown that \(X\) has a hyperelliptic descent if \(\overline{G}\) is not cyclic of order coprime to \(\mathrm{char}(K)\), and if it is, examples of \(X\) with no hyperelliptic descents were constructed in [C. J. Earle, in: Adv. Theory Riemann Surfaces, Proc. 1969 Stony Brook Conf., 119–130 (1971; Zbl 0218.32010)] and [G. Shimura, Nagoya Math. J. 45, 167–178 (1972; Zbl 0243.14012)]. Recently, found in [E. Bujalance and P. Turbek, Manuscr. Math. 108, No. 1, 1–11 (2002; Zbl 0997.14008)] is the full classification of hyperelliptic curves that has a hyperelliptic descent for \(\mathbb C/\mathbb R\).

In this paper under review the authors present a complete answer to the descent problem for the case where \(K/k\) is any extenstion and \(\overline{G}\) is cyclic of order coprime to \(\mathrm{char}(K)\). They prove that there is always a field extension \(L\) of \(k\) with minimal degree \([L:k]\leq 2\) such that \(X\) has a hyperelliptic model over \(L\), and they give us explicit conditions on determining when \([L:k]=1\) or \(2\). The paper also presents how a descent can be effectively constructed, and given a quadratic extension \(L/k\), it gives an explicit description of the \(K\)-isomorphism classes of the curves which are defined over \(L\) and \(K\)-isomorphic to their conjugates, but do not descend to \(k\).

If \(X\) is a hyperelliptic curve, the presence of the hyperelliptic involution \(\iota\) makes \(\mathrm{Aut}(X_K)\) nontrivial. If we further require the existence of a model in the form of a hyperelliptic equation \(y^2=p(x)\) over \(k\), we call such a model a hyperelliptic descent of \(X\). Let \(G=\mathrm{Aut}(X_K)\), and \(\overline{G}=G/\langle \iota \rangle\). In [B. Huggins, Math. Res. Lett. 14, No. 2, 249–262 (2007; Zbl 1126.14036)], it was shown that \(X\) has a hyperelliptic descent if \(\overline{G}\) is not cyclic of order coprime to \(\mathrm{char}(K)\), and if it is, examples of \(X\) with no hyperelliptic descents were constructed in [C. J. Earle, in: Adv. Theory Riemann Surfaces, Proc. 1969 Stony Brook Conf., 119–130 (1971; Zbl 0218.32010)] and [G. Shimura, Nagoya Math. J. 45, 167–178 (1972; Zbl 0243.14012)]. Recently, found in [E. Bujalance and P. Turbek, Manuscr. Math. 108, No. 1, 1–11 (2002; Zbl 0997.14008)] is the full classification of hyperelliptic curves that has a hyperelliptic descent for \(\mathbb C/\mathbb R\).

In this paper under review the authors present a complete answer to the descent problem for the case where \(K/k\) is any extenstion and \(\overline{G}\) is cyclic of order coprime to \(\mathrm{char}(K)\). They prove that there is always a field extension \(L\) of \(k\) with minimal degree \([L:k]\leq 2\) such that \(X\) has a hyperelliptic model over \(L\), and they give us explicit conditions on determining when \([L:k]=1\) or \(2\). The paper also presents how a descent can be effectively constructed, and given a quadratic extension \(L/k\), it gives an explicit description of the \(K\)-isomorphism classes of the curves which are defined over \(L\) and \(K\)-isomorphic to their conjugates, but do not descend to \(k\).

Reviewer: Sungkon Chang (Savannah)

##### MSC:

14Q05 | Computational aspects of algebraic curves |

13A50 | Actions of groups on commutative rings; invariant theory |

14H10 | Families, moduli of curves (algebraic) |

14H25 | Arithmetic ground fields for curves |

14H37 | Automorphisms of curves |

##### Software:

Magma##### References:

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