Renormalized Waves and Discrete Breathers in -Fermi-Pasta-Ulam Chains

0031-9007= We demonstrate via numerical simulation that in the strongly nonlinear limit the -Fermi-Pasta-Ulam ( -FPU) system in thermal equilibrium behaves surprisingly like weakly nonlinear waves in properly renormalized normal variables. This arises because the collective effect of strongly nonlinear interactions effectively renormalizes linear dispersion frequency and leads to effectively weak interaction among these renormalized waves. Furthermore, we show that the dynamical scenario for thermalized -FPU chains is spatially highly localized discrete breathers riding chaotically on spatially extended, renormalized waves.