1. N. N. Bogoljubov and D. V. Širkov, Introduction to the theory of quantized fields, Interscience, New York, 1959.
2. Eigenfunction expansions for non-symmetric partial differential operators. I;Browder, Felix E.;Amer. J. Math.,1958
3. American Mathematical Society Colloquium Publications, Vol. 31;Hille, Einar,1957
4. Perturbation of continuous spectra by unbounded operators. I;Kuroda, Shige Toshi;J. Math. Soc. Japan,1959
5. The scattering problem for non-stationary perturbations;Nižnik, L. P.;Soviet Math. Dokl.,1960