A formal theory of resonances is presented in the Feshbach formalism with the use of more general projection operators and the relationship with other resonance formulations is clarified. The resonance position and true width are defined. They depend upon the background, or potential, scattering. An attempt is made to define the resonance state without using a "preconceived notion" of its structure. The attempt is not totally successful but the resultant equation is shown to be useful in obtaining corrections to other resonance formulations.
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Marvin H. Mittleman (1969) studied this question.
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