Packets of resonant modes in the Fermi-Pasta-Ulam system

► We study the role of resonances for energy localization and metastability in the FPU model. ► We construct an approximated first integral associated to the metastable state. ► We propose to identify the metastable state as a packet of resonant modes. ► The packet extend up to a frequency ω⁎(ε), which depends only on the specific energy ε.