Bic\^{o}ne nilpotent d'une alg\`{e}bre de Lie r\'{e}ductive
  I prefer taking off this paper for the moment because of a mistake in the lemma 2.1 of the secund version. Precisely, in the proof of this lemma, it is not clear that the morphism $r\_j$ is flat, that I claim it. 
Anne Moreau (IMJ)