1. H. Leutwyler:Phys. Rev.,134, B 1155 (1965). This paper contains a more extensive list of the relevant literature and is quoted as I.
2. R. P. Feynman:Rer. Mod. Phys.,20, 267 (1948).
3. R. Arnowitt, S. Deser andC. W. Misner:Phys. Rev.,113, 745 (1959);116, 1322 (1959);117, 1595 (1960);118, 1100 (1960);R. F. Baierlein, D. H. Sharp andJ. A. Wheeler:Phys. Rev.,126, 1864 (1962);J. A. Wheeler inChiu andHoffmann:Gravitation and Relativity (New York, 1964).
4. We shall assume either that the surfacesx o=constant are closed and that therefore no such boundary conditions are needed or that sufficiently strong conditions for the behaviour of the space at spacelike infinity provide the necessary boundary conditions. See also ref
5. G. Darboux:Leçons sur la théorie générale des surfaces, III (Paris, 1894);L. Bianchi:Vorlesungen über Differentialgeometrie, chapt. VII (Leipzig, 1910);A. R. Forsyth:Lectures on Differential Geometry, chapt. X (Cambridge, 1920);W. Blaschke:Differentialgeometrie, II (Berlin, 1923).