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2. The analytic continuation to the unphysical sheet of the resolvent kernel and the scattering operator associated with the Schrôdinger operator;Dolph;J. Math. Anal. Appl.,1966
3. Causality and analyticity;Fourès;Trans. Am. Math. Soc.,1955
4. Wave operators and unitary equivalence;Kato;Pacific J. Math.,1965
5. Spectral theory of operators in spaces with an indefinite metric. I and II;Iohvidov,1956