Modulational Estimate for Fermi-Pasta-Ulam Chain Lyapunov Exponents

In the framework of the Fermi-Pasta-Ulam (FPU) model, we show a simple method to give an accurate analytical estimation of the maximal Lyapunov exponent at high energy density. The method is based on the computation of the mean value of the modulational instability growth rates associated to unstable modes. Moreover, we show that the strong stochasticity threshold found in the $\beta$-FPU system is closely related to a transition in tangent space: the Lyapunov eigenvector being more localized in space at high energy.