Abstract
By means of the two-particle Green function, the collective modes of a condensed Fermi system (i.e. a Fermi system with long-range order present) describable by means of a 2*2 matrix propagator are studied. The random phase approximation is given in terms of diagrams and is applied to a ferromagnetic state. Finally, the Goldstone theorem is shown to be an immediate consequence of the order-parameter equation.