1. Bramble, J. H., Zlamal, M.: Maths. Comp.24, 809?820 (1970)
2. Lehman, R. S.: J. Math. Mech.8, 727?760 (1959)
3. Motz, H.: Q. Ap. Mech.4, 371?390 (1946)
4. Smirnov, V. I.: A Course in Higher Mathematics. Vol. 5, London: Perg. Press 1964
5. Strang, G., Fix, G.: An Analysis of the Finite Element Method: Englewoods Cliffs N. J.: Prentice Hall 1973