Un théorème de Rao pour les familles de courbes gauches

Abstract

The aim of this paper is to prove a generalization of a theorem of Rao for families of space curves, which caracterizes the biliaison classes of curves. First we introduce the concept of pseudo-isomorphism of coherent sheaves, which generalizes the concept of stable isomorphism. An N-type resolution for a family of curves $C$ over the local ring $A$, defined by an ideal $J$, is an exact sequence $0\to P\to N\to J\to 0$ where $N$ is a locally free sheaf on $P_A^3$ and $P$ is a direct sum of invertible sheaves $O_P(-n_i)$. We prove the two following results, when the residual field of $A$ is infinite : 1. Let $C$ and $C^{\prime }$ be two flat families of space curves over the local ring $A$. Then $C$ and $C^{\prime }$ are in the same biliaison class if and only if their ideals $J$ and $J^{\prime }$ are pseudo-isomorphic, up to a shift. 2. Let $C$ and $C^{\prime }$ be two flat families of space curves over the local ring $A$, with N-type resolutions, involving sheaves $N$ and $N^{\prime }$. Then $C$ and $C^{\prime }$ are in the same biliaison class if and only if $N$ and $N^{\prime }$ are pseudo-isomorphic, up to a shift.

22 pages, TeX