Relationship between the Schwinger and Kohn-type variational principles in scattering theory

We establish the correct mathematical relationship between the Schwinger and Kohn variational principles for scattering theory and show that the Schwinger principle is one rank higher than the Kohn principle. If the same trial scattering wave function is used in these two principles, the Schwinger method should hence give superior results. Application of the Schwinger and Kohn variational principles to scattering by a simple model potential gives results which clearly illustrate this relationship.