  We study the effect of interactions on the zero-temperature a.c. conductivity
of 2D Anderson insulator at low frequencies. We show that the enhancement of
the real part of conductivity due to the Coulomb correlations in the occupation
numbers of localized states results in the change of the sign of imaginary part
within a certain frequency range. As a result, the propagation of a surface
plasmon in a localized system becomes possible. We analize the dispersion law
of the plasmon for the two cases: unscreened Coulomb interactions and the
interactions screened by a gate electrode spaced by some distance from the
electron plane.
