Multiple singular values of Hankel operators

The goal of this paper is to construct a nonlinear Fourier transformation on the space of symbols of compact Hankel operators on the circle. This transformation allows to solve a general inverse spectral problem involving singular values of a compact Hankel operator, with arbitrary multiplicities. The formulation of this result requires the introduction of the pair made with a Hankel operator and its shifted Hankel operator. As an application, we prove that the space of symbols of compact Hankel operators on the circle admits a singular foliation made of tori of finite or infinite dimensions, on which the flow of the cubic Szeg\"o equation acts. In particular, we infer that arbitrary solutions of the cubic Szeg\"o equation on the circle with finite momentum are almost periodic with values in H^{1/2}(S ^1).