1. R. Schechter:The Variational Method in Engineering (New York, N. Y., 1967).
2. B. Vujanovic:AIAA Journ.,9, 131 (1971).
3. B. Vujanovic andD. J. Djukic:Int. Journ. Heat Mass Transfer,15, 1111 (1972).
4. H. Helmholtz:Journ. reine angew. Math.,100, 137 (1887).
5. A. Mayer:Ber. Kon. Ges. Wiss. Leipzig, Math.-Phys. Kl.,48, 519 (1896).