Validity of Certain Soft Photon Amplitudes
  Certain soft photon amplitudes which have been recently suggested as alternatives to the usual Low form of the soft photon approximation are studied and it is demonstrated that problems exist in their relation to the corresponding non-radiative amplitude. The non-radiative amplitude, which is an input to soft photon calculations, is in certain cases required to be evaluated outside of its physical phase space region. Also, for the case of two-body identical particle bremsstrahlung processes, the symmetrized or antisymmetrized form of these soft photon amplitudes cannot be written in terms of the symmetrized or antisymmetrized amplitude for the non-radiative process. It is found that the usual Low form of the soft photon theorem is essentially unaffected by these problems. 
Mark Welsh (Univ. of Victoria), Harold W. Fearing (TRIUMF)