Abstract
We reconsider the phenomenon of localization of energy in low frequency modes in the FPU system, exploiting the resonances in the lower part of the spectrum. Using the resonant normal form of Birkhoff we construct some candidates of approximate first integrals which we put in correspondence to packets of low frequency modes. By numerical calculation we show that the packet associated to the best quasi-integral involves all modes up to a frequency ω⁎(ε), where ε is the specific energy. The phenomenon disappears when the specific energy is bigger than a threshold value. The dependence of the relevant quantities on the number N of particles is also investigated. A final section is devoted to a first comparison with the Toda model.
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