1. Einstein, A., König. Preuss. Akad. der Wissenschaften, Sitzungsberichte, Erster Halbband, 688 (1916), and 154 (1918).
2. Einstein, A., König. Preuss. Akad. der Wissenschaften, Sitzungsberichte, zweiter Halbband, 833 (1915); 844 (1915).
3. Our notations differ from those of Einstein because as fourth space-time coordinate he uses the imaginary variable x4 = it, while we use the real variable xo = ct. Furthermore our hik corresponds to his γμ/v.
4. Levi-Civita, Rend. Ace. Lincei
26, 381.... (1917)
5. See, for example: Landau, L.D., and Lifshitz, E.M., The classical theory of fields, Pergamon, Oxford (1971).