1. On the search for variational principles
2. 2Lanczos, C., The Variational Principles of Mechanics, 3rd Ed.University of Toronto Press, Toronto, 1966, pp.70-72.
3. The method of Ritz applied to the equation of Hamilton
4. 4Zienkiewicz, O. C., The Finite Element Method, 3rd Ed.McGraw-Hill, New York, 1977, pp.569-570.
5. 5Kantorovich, L. V. and Krylor, V. L, Approximate Methods of Higher Analysis, 3rd Ed.Interscience Publishers, Inc., The Netherlands, 1964, pp.258-303.