Pion re-scattering operator in the S-matrix approach
  The pion re-scattering operator for pion production, derived recently in time-ordered perturbation theory, is compared with the one following from the simple S-matrix construction. We show that this construction is equivalent to the on-shell approximation introduced in previous papers. For a realistic NN interaction, the S-matrix approach, and its simplified fixed threshold-kinematics version, work well near threshold. 
V. Malafaia, J. Adam Jr and M. T. Pena