Jean-Marc Delort – författare

The goal of this monograph is to prove that any solution of the Cauchy problem for the capillary-gravity water waves equations, in one space dimension, with periodic, even in space, small and smooth enough initial data, is almost globally defined in time on Sobolev spaces, provided the gravity-capillarity parameters are taken outside an exceptional subset of zero measure. In contrast to the many results known for these equations on the real line, with decaying Cauchy data, one cannot make use of dispersive properties of the linear flow. Instead, a normal forms-based procedure is used, eliminating those contributions to the Sobolev energy that are of lower degree of homogeneity in the solution. Since the water waves equations form a quasi-linear system, the usual normal forms approaches would face the well-known problem of losses of derivatives in the unbounded transformations. To overcome this, after a paralinearization of the capillary-gravity water waves equations,we perform several paradifferential reductions to obtain a diagonal system with constant coefficient symbols, up to smoothing remainders. Then we start with a normal form procedure where the small divisors are compensated by the previous paradifferential regularization. The reversible structure of the water waves equations, and the fact that we seek solutions even in space, guarantees a key cancellation which prevents the growth of the Sobolev norms of the solutions.

This text grew up from lecturcs givcn at he t University of Rennes I during the academic year 1988-1989. The main topics covered arc second microlocalization along a agrangian l manifold, defined by Sjostrand in [Sj], and its application to the study of conormal sin- gularities for solutions of semilinear hyperbolic partial differential equations, developed by Lebeau [L4]. To give a quite self-contained treatment of these questions, we induded some de- velopments about FBI transformations and subanalytic geometry. The text is made oi four chapters. In he t first one, we define the Fourier-Bros-Ingolnitzer transionnation and study its main properties. The second chapter deals with second microlocalization along a lagrangian submanifold, and with upper bounds for the wave front set of traces one may obtain using it. The third chapter is devoted to formulas giving geometric upper bounds for the analytic wave front set and for the ser,ond mic:rosllpport of boundary values of ramified functions.Lastly, the fourth chapter applies the preceding methods to the derivation of theorems about the location of microlocal singularities of solutions of scmilinear wave equations with conormw data, in general geometrical situation. Every chapter begins with a short abstract of its contents, where are collected the bibliograph- ical references. Let me now thank all those who made this writing possible. First of all, Gilles Lebeau, from whom I learnt mcrol i ocal analysis, especially through lectures he gave with Yves Laurent at Ecole Normale Superieure in 1982-1983.