1. B. Bertotti:Nuovo Cimento,4, 898 (1956);6, 755 (1957).
2. P. T. Hoang:Compt. Rend.,246, 61 (1958).
3. N. Rosen:Phys. Rev.,57, 147 (1940).
4. V. A. Fock:Theory of space, time and gravitation (in Russian, Moscow, 1955), p. 441.
5. L. Landau andE. Lifshitz:The Classical Theory of Fields (English translation, Cambridge, Mass., 1951), p. 324.