Abstract-In this phase of the attempt to advance finite dimensional algebraic function approximation technique in eigenvalue problems of lossless metallic guides filled with anisotropic and/or inhomogeneous media, to exact analysis in infinite dimensions, it is seen that the problem in infinite dimensions, can be reduced to finite dimensions, by virtue of a result in perturbation theory. Furthermore, it is found that analysis results of algebraic function approximation, can be adapted to infinite dimensions too, at worst by introduction of some additional arguments.