1. Timoshenko S., Krieger W.: Theory of Plates and Shells. Mc Graw Hill, New York (1959)
2. Guelfand I., Fomin S.: Calculus of Variations. Prentice Hall, New Jersey (1963)
3. Kantorovich L., Krylov V.: Approximate Methods of Higher Analysis. Interscience Publishers, New York (1964)
4. Mikhlin S.: Variational Methods of Mathematical Physics. Mac Millan, New York (1964)
5. Hildebrand F.: Methods of Applied Mathematics. Prentice Hall, New Jersey (1965)