1. H. Hönl, A. Maue, and K. Westpfahl, Diffraction Theory [Russian translation], Izd. Mir, Moscow (1964).
2. S. G. Mikhlin, Variational Methods in Mathematical Physics [in Russian], Izd. Nauka, Moscow (1970).
3. V. V. Nikol'skii, Variational Methods for Internal Problems of Electrodynamics [in Russian], Izd. Nauka, Moscow (1967).
4. L. V. Kantorovich and V. I. Krylov, Approximate Methods of Higher Analysis [in Russian], Fizmatgiz, Moscow (1962).
5. C. Muller, Grundlagen der Mathematischen Theorie Electromagnetischen Schwingungen, Berlin (1957).