Mathematics > Algebraic Geometry
[Submitted on 18 Jul 2018 (v1), last revised 29 Aug 2019 (this version, v3)]
Title:Halves of points of an odd degree hyperelliptic curve in its jacobian
View PDFAbstract:Let $f(x)$ be a degree $(2g+1)$ monic polynomial with coefficients in an algebraically closed field $K$ with $char(K)\ne 2$ and without repeated roots. Let $\mathfrak{R}\subset K$ be the $(2g+1)$-element set of roots of $f(x)$. Let $\mathcal{C}: y^2=f(x)$ be an odd degree genus $g$ hyperelliptic curve over $K$. Let $J$ be the jacobian of $\mathcal{C}$ and $J[2]\subset J(K)$ the (sub)group of its points of order dividing $2$. We identify $\mathcal{C}$ with the image of its canonical embedding into $J$ (the infinite point of $\mathcal{C}$ goes to the identity element of $J$). Let $P=(a,b)\in \mathcal{C}(K)\subset J(K)$ and $M_{1/2,P}\subset J(K)$ the set of halves of $P$ in $J(K)$, which is $J[2]$-torsor. In a previous work we established an explicit bijection between $M_{1/2,P}$ and the set of collections of square roots $$\mathfrak{R}_{1/2,P}:=\{\mathfrak{r}: \mathfrak{R} \to K\mid \mathfrak{r}(\alpha)^2=a-\alpha \ \forall \alpha\in\mathfrak{R}; \
\prod_{\alpha\in\mathfrak{R}} \mathfrak{r}(\alpha)=-b\}.$$ The aim of this paper is to describe the induced action of $J[2]$ on $\mathfrak{R}_{1/2,P}$ (i.e., how signs of square roots $\mathfrak{r}(\alpha)=\sqrt{a-\alpha}$ should change).
Submission history
From: Yuri Zarhin G. [view email][v1] Wed, 18 Jul 2018 15:48:21 UTC (13 KB)
[v2] Sun, 3 Feb 2019 00:45:44 UTC (13 KB)
[v3] Thu, 29 Aug 2019 16:53:21 UTC (13 KB)
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