2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/229597Within the relativistic quantum field theory, we analyze the differences between the $πN$ reaction models constructed from using (1) three-dimensional reductions of Bethe-Salpeter Equation, (2) method of unitary transformation, and (3) time-ordered perturbation theory. Their relations with the approach based on the dispersion relations of S-matrix theory are dicusssed.Nuclear Theorytext