Abstract
Green functions (GF) containing spin operators have their own peculiarities owing to the commutation relations of spin operators, which are neither Bose nor Fermi operators. In a perturbation theory the resulting series become even more difficult than in the Bose or Fermi case. In particular, Dyson’s equation is not sufficient for summing all the contributions appearing in the perturbation series. After some contradicting opinions the present answer to this problem is the following.
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