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 be a smooth projective algebraic curve of genus and an integer with . For all integers we prove the existence of a double covering with a smooth curve of genus and the existence of a degree morphism that does not factor through . By the Castelnuovo-Severi inequality, the result is sharp (except perhaps the bound ).