Abstract
A self-consistent many-body theory for the many-particle Green's function is developed and applied to derive an exact microscopic formula for the transverse dynamical susceptibility. As a concrete application of the above general results we consider the Hubbard Hamiltonian and obtain the spin-wave stiffness constant for an itinerant ferromagnet. It is found that earlier expressions for the stiffness constant obtained by Edwards and Hertz and Edwards are special cases of our general expression.

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