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Deformation of finite morphisms and smoothing of ropes

Published online by Cambridge University Press:  01 May 2008

Francisco Javier Gallego
Affiliation:
Departamento de Álgebra, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain (email: gallego@mat.ucm.es, mgonza@mat.ucm.es)
Miguel González
Affiliation:
Departamento de Álgebra, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain (email: gallego@mat.ucm.es, mgonza@mat.ucm.es)
Bangere P. Purnaprajna
Affiliation:
Department of Mathematics, University of Kansas, Snow Hall, 1460 Jayhawk Boulevard, Lawrence, KS 66045-7523, USA (email: purna@math.ku.edu)
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Abstract

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In this paper we prove that most ropes of arbitrary multiplicity supported on smooth curves can be smoothed. By a rope being smoothable we mean that the rope is the flat limit of a family of smooth, irreducible curves. To construct a smoothing, we connect, on the one hand, deformations of a finite morphism to projective space and, on the other hand, morphisms from a rope to projective space. We also prove a general result of independent interest, namely that finite covers onto smooth irreducible curves embedded in projective space can be deformed to a family of 1:1 maps. We apply our general theory to prove the smoothing of ropes of multiplicity 3 on P1. Even though this paper focuses on ropes of dimension 1, our method yields a general approach to deal with the smoothing of ropes of higher dimension.

Type
Research Article
Copyright
Copyright © Foundation Compositio Mathematica 2008