1. E. Noether:Nachr. Ges. Göttingen, Math. Phys. Kl. (1919), p. 235.
2. For a frank, but somewhat biased statement of the worth of variational principles as applied to continuum mechanics see (4).
3. D. G. B. Edelen:Arch. Rat. Mech. Anal.,5, 95 (1960).
4. J. A. Schouten:Ricci-Calculus, 2nd ed. (Berlin, 1954).
5. C. Truesdell andR. Toupin:The classical field theories inHandb. der Phys., vol.3/1 (Berlin, 1960), p. 594.