1. See inE. T. Whittacker andG. N. Watson:A Course of Modern Analysis (Cambridge, London and New York, N. Y., 1969), p. 400.
2. R. Courant andD. Hilbert:Methods of Mathematical Physics, Vol.2 (New York, N. Y., 1962).
3. A. Trautman:Proc. Roy. Soc., A270, 326 (1962) (here a private communication ofI. Robinson is acknowledged).
4. E. T. Newman andJ. Winicour:Journ. Math. Phys.,15, 426 (1974);R. W. Lind andE. T. Newman:Journ. Math. Phys.,15, 1103 (1974);E. T. Newman andJ. Winicour:Journ. Math. Phys.,15, 113 (1974).
5. E. T. Newman: inTel Aviv GR 7 Lecture (1974); in talks atBergmann's Symposium in Syracuse (1975); atMathematical Physics Symposium in México (1975);