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Faisceaux cohérents sur les courbes multiples.

Jean-Marc Drézet (2006)

Collectanea Mathematica

This paper is devoted to the study of coherent sheaves on non reduced curves that can be locally embedded in smooth surfaces. If Y is such a curve then there is a filtration C ⊂ C2 ⊂ ... ⊂ Cn = Y such that C is the reduced curve associated to Y, and for very P ∈ C there exists z ∈ OY,P such that (zi) is the ideal of Ci in OY,P. We define, using canonical filtrations, new invariants of coherent sheaves on Y: the generalized rank and degree, and use them to state a Riemann-Roch theorem for sheaves...

Families of curves and alterations

A. Johan De Jong (1997)

Annales de l'institut Fourier

In this article it is shown that any family of curves can be altered into a semi-stable family. This implies that if S is an excellent scheme of dimension at most 2 and X is a separated integral scheme of finite type over S , then X can be altered into a regular scheme. This result is stronger then the results of [ Smoothness, semi-stability and alterations to appear in Publ. Math. IHES]. In addition we deal with situations where a finite group acts.

Families of elliptic curves with genus 2 covers of degree 2.

Claus Diem (2006)

Collectanea Mathematica

We study genus 2 covers of relative elliptic curves over an arbitrary base in which 2 is invertible. Particular emphasis lies on the case that the covering degree is 2. We show that the data in the "basic construction" of genus 2 covers of relative elliptic curves determine the cover in a unique way (up to isomorphism).A classical theorem says that a genus 2 cover of an elliptic curve of degree 2 over a field of characteristic ≠ 2 is birational to a product of two elliptic curves over the projective...

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