S-matrix approach to hadronic atom level shifts

$S$-matrix perturbation theory accurate to all orders in the strong interaction potential is developed. The method presented is applied to the derivation of two important relations: (a) The complex level shift in a hadronic atom is expressed in terms of the logarithmic derivative of the regular wave function at the nuclear boundary; and (b) the complex level shift is related in a model independent way to the zero-energy hadron-nucleus scattering phase shift. It is shown that these two relations provide practical means for a fast and accurate evaluation of the complex level shifts and generalized scattering lengths.