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The main results of the picture of itinerant ferromagnetism previously presented are obtained by the functional-integral method of Stratonovitch and Hubbard. A new form of this technique is introduced which is spin rotationally invariant in all approximations and which respects the Pauli principle. By a generalization of the saddle-point method, the itinerant-electron thermodynamics is transformed to that of a classical Heisenberg model with Ruderman-Kittel exchange.
Prange et al. (Tue,) studied this question.