Birkhoff coordinates for the Toda lattice in the limit of infinitely many particles with an application to FPU

In this paper we study the Birkhoff coordinates (Cartesian action angle coordinates) of the Toda lattice with periodic boundary condition in the limit where the number N   of the particles tends to infinity. We prove that the transformation introducing such coordinates maps analytically a complex ball of radius R/NαR/Nα (in discrete Sobolev-analytic norms) into a ball of radius R′/NαR′/Nα (with R,R′>0R,R′>0 independent of N  ) if and only if α≥2α≥2. Then we consider the problem of equipartition of energy in the spirit of Fermi–Pasta–Ulam. We deduce that corresponding to initial data of size R/N2R/N2, 0