1. A. J. W. Sommerfeld,Ann. d. Physik 50, 1 (1916).
2. C. G. Darwin,Proc. Roy. Soc. 118, 654 (1928); W. Gordon,Z. f. Physik 48, 11 (1928).
3. W. Yourgrau and S. Mandelstam,Variational Principles in Dynamics and Quantum Theory (W. B. Saunders Co., Philadelphia, 1968), 3rd ed.
4. Letter to Yourgrau-Mandelstam, dated 29/II/56 and quoted, loc. cit. ref. (3), p. 113 ff. The italics are in the original.
5. Loc. cit. (Ref. 3 ), p. 115.