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1983.
Perturbation expansion of S matrix for background scattering
The S matrix near a pole is parametrized into the contribution from the resonance and the background scattering. We develop a perturbation theory for the background scattering, based on the Jost function formalism. A closed expression is found up to the second order in the coupling strength between the channels. A brief comparison with the other formalism is also made and the advantages of the present theory are shown.