1. A. Ya. Povzner, "Expansion of arbitrary functions in terms of the eigenfunctions of the operator −Δu + qu," Matem. Sb.,32, 109–156 (1953).
2. T. Ikebe, "Eigenfunction expansions associated with the Schrödinger operator and their applications to scattering theory," Arch. Rat. Mech. Anal.,5, 1–34 (1960).
3. L. D. Faddeev, "The Friedrichs model in the theory of perturbations of continuous spectra," Trudy MIAN,73, 292–313 (1964).
4. L. P. Nizhnik, "Spectral properties of self-adjoint partial differential operators close to operators with constant coefficients," Material from the Soviet-American Symposium, Novosibirsk (1963).
5. L. P. Nizhnik, "The scattering problem for a single Schrödinger equation," Ukr. Matem. Zh., No. 2, 209–212 (1960).