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Double covering of curves
Authors:Edoardo Ballico   Changho Keem   Seungsuk Park
Affiliation:Department of Mathematics, Università di Trento, 38050 Povo(TN), Italy ; Department of Mathematics, Seoul National University, Seoul 151-742, South Korea ; Department of Mathematics, Seoul National University, Seoul 151-742, South Korea
Abstract:Let $C$ be a smooth projective algebraic curve of genus $q$ and $g$ an integer with $gge 4q+5$. For all integers $dge g-2q+1$ we prove the existence of a double covering $f:Xto C$with $X$ a smooth curve of genus $g$ and the existence of a degree $d$ morphism $u:Xto mathbb P^1$ that does not factor through $f$. By the Castelnuovo-Severi inequality, the result is sharp (except perhaps the bound $gge 4q+5$).

Keywords:Double coverings   base-point-free pencil   Castelnuovo-Severi inequality   Brill-Noether theory
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