  A methodical derivation of RKKY interaction in framework of T=0 Green
function method is given in great detail. The article is complimentary to
standard textbooks on the physics of magnetism and condensed matter physics. It
is shown that the methods of statistical mechanics gives a standard and
probably simplest derivation of the exchange interaction. A parallel with
theory of plasma waves demonstrates the relation between the Fourier
transformation of polarization operator of degenerate electron gas at zero
frequency and the space dependence of the indirect electron exchange due to
itinerant electrons.
