  We present a theoretical treatment of light scattering by an ensemble of N
dipoles, taking into account recurrent multiple scattering. We study the
intrinsic optical properties of collective dipolar systems without specifying a
particular illumination condition. We apply this formalism to study the
collective absorption modes for an ensemble of small nanoparticles and then to
derive collective radiative corrections due to mutual interactions in dipolar
ensembles, a topic of major importance in the development of collective
nanophotonic systems or atomic networks used as optical clocks.
