An Averaging Theorem for FPU in the Thermodynamic Limit

Consider an FPU chain composed of N≫1 particles, and endow the phase space with the Gibbs measure corresponding to a small temperature β-1. Given a fixed K, we construct K packets of normal modes whose energies are adiabatic invariants (i.e., are approximately constant for times of order β1-a, a>0) for initial data in a set of large measure. Furthermore, the time autocorrelation function of the energy of each packet does not decay significantly for times of order β. The restrictions on the shape of the packets are very mild. All estimates are uniform in the number N of particles and thus hold in the thermodynamic limit N→∞, β>0.