Higher approximations to magnon renormalization

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Abstract
A brief review of the history of the concept of magnon renormalization in a Heisenberg ferromagnet is given. An algebraically simple scheme for calculating renormalized energies is developed, and relations of this scheme to previous methods are explained. The results depart only slightly from the method of Bloch, with the exception that the lower branch appears to take on meaning. The relation to Green's-function methods is established through the moments of the spectral weighting function.