MODULATIONAL ESTIMATE FOR THE MAXIMAL LYAPUNOV EXPONENT IN FERMI-PASTA-ULAM CHAINS

In the framework of the Fermi-Pasta-Ulam (FPU) model, we show a simple method to give an accurate analytical estimation of the maximal Lyapu nov exponent at high energy density. The method is based on the comput ation of the mean value of the modulational instability growth rates a ssociated to unstable modes. Moreover. we show that the strong stochas ticity threshold found in the beta-FPU system is closely related to a transition in tangent space, the Lyapunov eigenvector being more local ized in space at high energy.