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Some extremal contractions between smooth varieties arising from projective geometry

Published online by Cambridge University Press:  30 June 2004

Alberto Alzati
Affiliation:
Dipartimento di Matematica ‘F. Enriques’, Università degli Studi di Milano, Via C. Saldini 50, 20133 Milan, Italy. E-mail: [email protected]
Francesco Russo
Affiliation:
Departamento de Matematica, Universidade Federal de Pernambuco, Cidade Universitaria, 50670-901 Recife–PE, Brazil. E-mail: [email protected]
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Abstract

We construct explicit examples of elementary extremal contractions, both birational and of fiber type, from smooth projective $n$-dimensional varieties, with $n \geq 4$, onto smooth projective varieties, arising from classical projective geometry and defined over sufficiently small fields, not necessarily algebraically closed.

The examples considered come from particular special homaloidal and subhomaloidal linear systems, which are usually degenerations of general phenomena classically investigated by Bordiga, Severi, Todd, Room, Fano, Semple and Tyrrell and more recently by Ein and Shepherd-Barron.

The first series of examples is associated to particular codimension 2 determinantal smooth subvarieties of $\mathbf{P}^{m}$, with $3 \leq m \leq 5$. We get another series of examples by considering special cubic hypersurfaces through some surfaces in $\mathbf{P}^5$, or some 3-folds in $\mathbf{P}^7$ having one apparent double point. The last examples come from an intriguing birational elementary extremal contraction in dimension 6, studied by Semple and Tyrrell and fully described in the last section of the paper.

Type
Research Article
Copyright
2004 London Mathematical Society

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Footnotes

The research of the first author was partially supported by the M.I.U.R. of the Italian Government in the framework of the national Research Project ‘Geometry of algebraic varieties’ (Cofin 2002).
The research of the second author was partially supported by CNPq (Centro Nacional de Pesquisa), grant 300761/97-0 and by PRONEX-Algebra Comutativa e Geometria Algebrica.