Variational stability and the form of the Schwinger multichannel variational principle

We derive the Schwinger multichannel variational principle [K. Takatsuka and V. McKoy, Phys. Rev. A 23, 2352 (1981); 30, 1734 (1984)] by a method that emphasizes the question of variational stability. The present approach is in some respects more direct than the original method and serves to clarify the role of a certain parameter that occurs in the formulation