An operator formalism of the general single-channel Kohn method is developed, and Schwartz's theory of singularities in the Kohn method is generalized; any approximation method described in the framework of the Kohn method is shown to have, in general, singularities, some of which may correspond to true (either closed-channel or shape) resonances. Caution is urged against simple interpretation of “resonance results” of close-coupling calculations and other expansion methods. The Feshbach Q H Q eigenvalues are shown to be approximations to the positions of the Kohn-method singularities. A correction to the Schwartz theory leads to the Harris method. Precisely at the characteristic Harris eigenvalues, the Hulthen condition \(\langle\varPsi|E-H|\varPsi\rangle{=}0\) cannot be satisfied in general, though, as was shown previously, a well-defined limiting value (which is equal to the Harris value, and has a first-order error contrary to a variational phase shift) of the Hulthen phase shift exists. An elaborate ...