  Plasmon spectrum and polarization operator of 1, 2, and 3 dimensional
electron gas are calculated by T=0 Green function technique. It is shown that
this field theory method gives probably the simplest pedagogical derivation of
the statistical problem for the response function. The explanation is
complimentary to the standard courses on condensed matter and plasma physics of
the level of IX volume of Landau-Lifshitz encyclopedia on theoretical physics.
