  We give a derivation of the particle-hole RPA equations for an interacting
multi-fermion system by applying the instantaneous approximation to the
amputated two-fermion propagator of the system. In relativistic field theory
the same approximation leads from the fermion-antifermion Bethe-Salpeter
equation to the Salpeter equation. We show that RPA equations and Salpeter
equation are indeed equivalent.
