1. Birman, M. S.: Variation methods of solving boundary problems, analogous Trefftz methods. [in Russian]. Viestnik Leningradskowo Universiteta Nr 13 1956
2. Collatz, L.: Numerische Behandlung von Differentialgleichungen. Berlin, Göttingen, Heidelberg: Springer 1955
3. Collin, R. E.: Field Theory of Guided Waves. New York: McGraw-Hill 1960
4. Kantorovich, L. W.; Krylov, W. I.: Approximate methods of analysis [in Russian]. Moscow 1950
5. Mikhlin, S. G.: Variation Methods in Mathematical Physics [in Russian]. Moscow 1970