Quasipotential approach to scattering theory: Spectral analysis of the vertex function

The vertex function of the quasipotential in scattering equations is spectral analyzed in terms of the strength eigenstates, and a criterion for its effectiveness established. A simple variational procedure is given to construct the strength eigenstates, by which vertex functions may be evaluated.NUCLEAR REACTIONS Quasipotential method, convergence of the iteration series.