On the Korteweg-de Vries long-wave approximation of the Gross-Pitaevskii   equation I
  The fact that the Korteweg-de-Vries equation offers a good approximation of long-wave solutions of small amplitude to the one-dimensional Gross-Pitaevskii equation was derived several years ago in the physical literature. In this paper, we provide a rigorous proof of this fact, and compute a precise estimate for the error term. Our proof relies on the integrability of both the equations. In particular, we give a relation between the invariants of the two equations, which, we hope, is of independent interest. 
Fabrice Bethuel (LJLL), Philippe Gravejat (CEREMADE), Jean-Claude Saut   (LM-Orsay), Didier Smets (LJLL)