1. Birman, M. Sh.: A local test of the existence of wave operators. Dokl. Akad. Nauk SSSR 159, 485–488 (1964) [Russian].
2. Birman, M. Sh., and S. B. Entina: Stationary approach in the abstract theory of scattering. Dokl. Akad. Nauk SSSR 155, 506–508 (1964); Izv. Akad. Nauk SSSR 31, 401–430 (1967) [Russian].
3. Faddeev, L. D.: On the Friedrichs model in the perturbation theory of continuous spectrum. Trudy Mat. Inst. Steklov. 73, 292–313 (1964) [Russian].
4. Friedrichs, K. O.: Über die Spektralzerlegung eines Integraloperators. Math. Ann. 115, 249–272 (1938).
5. Friedrichs, K. O.: On the perturbation of continuous spectra, Comm. Pure Appl. Math. 1, 361–406 (1948).