Perturbation Methods in Dispersion Theory

A complete perturbation method for $\frac{N}{D}$ calculations in potential scattering is presented. This method agrees with the Dashen-Frautschi method in first order. It is shown that in cases where resonant states are shifted by the perturbation into bound states the effect is not of first order and the Dashen-Frautschi method is therefore inadequate. Some possible utilizations of the perturbation method are outlined.