In core-collapse supernovae, the neutrino density is high enough to render
the nu-nu interactions not negligible. In particular, they can couple
the flavor evolution of neutrinos and induce collective flavor changes.
We discuss the most important feature observable in the energy spectra
(the so called spectral split), both in the case of luminosity equipartition
among flavors and for unconstrained luminosities. The spectral
split pattern is shown to depend strongly on the initial luminosity for
each flavor and the neutrino mass hierarchy. Pure collective three-flavor
effects wil be also analyzed.
