1. C. Misner, K. Thorne andJ. Wheeler:Gravitation (San Francisco, Cal., 1973).
2. A. Palatini:Rend. Circ. Math. Palermo,43, 203 (1919).
3. L. Landau andE. Lifshitz:Classical Theory of Fields, 3rd edition (Reading, Mass., 1971), p. 273.
4. One example where this occurs is for a perfect fluid, see for exampleJ. Ray:Journ. Math. Phys.,13, 1453 (1972).
5. D. Hilbert:Gottinger Nachr. (1915).