PERTURBATION THEORY FOR SPIN-1/2 HEISENBERG FERROMAGNET .I. WICKS THEOREM AND DIAGRAMMATIC REPRESENTATION

Pauli operators are treated as Bose operators fulfilling q-number commutation relations. An analogue of Wick's theorem for Pauli operators is formulated suitable for perturbation theory. The lowest-order contributions to the perturbation series for the temperature dependent one-particle causal Green function are calculated, and a diagrammatic representation is introduced for them. It is not necessary to have a different meaning of the same graph depending on the order of temperatures contained in it. The connected graphs representing the perturbation series up to second order are drawn explicitly.