The Yang-Feldman's method is generalized in order to obtain a covariant Hamiltonian formulation of quantized fields with non-local interactions. The interaction Hamiltonian is constructed according to the perturbation theory in such a way that the equation of motion in the interaction representation is integrable. The calculations are carried out actually up to the fourth order approximation.
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Chûshirô Hayashi (1953) studied this question.