Key points are not available for this paper at this time.
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.
Chûshirô Hayashi (Sun,) studied this question.