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N-Covers of hyperelliptic curves

Published online by Cambridge University Press:  02 May 2003

N. BRUIN
Affiliation:
Department of Mathematics & Statistics, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6. e-mail: bruin@cecm.sfu.ca
E. V. FLYNN
Affiliation:
Department Mathematical Sciences, University of Liverpool, Liverpool L69 3BX. e-mail: evflynn@liverpool.ac.uk

Abstract

For a hyperelliptic curve ${\hbox{\ax C}}$ of genus $g$ with a divisor class of order $n = g + 1$, we shall consider an associated covering collection of curves ${\hbox{\ax D}}_\delta$, each of genus $g^2$. We describe, up to isogeny, the Jacobian of each ${\hbox{\ax D}}_\delta$ via a map from ${\hbox{\ax D}}_\delta$ to ${\hbox{\ax C}}$, and two independent maps from ${\hbox{\ax D}}_\delta$ to a curve of genus $g(g-1)/2$. For some curves, this allows covering techniques that depend on arithmetic data of number fields of smaller degree than standard 2-coverings; we illustrate this by using 3-coverings to find all ${\Bbb Q}$-rational points on a curve of genus 2 for which 2-covering techniques would be impractical.

Type
Research Article
Copyright
2003 Cambridge Philosophical Society

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