1. V. De Sabbata andM. Gasperini:Nonlinear electromagnetic effects induced in vacuum by the gravitational interaction (submitted toLett. Nuovo Cimento).
2. See, for example,C. Isham, A. Salam andJ. Strathdee:Phys. Rev. D,3, 867 (1971).
3. K. Tennakone:Phys. Rev. D,10, 1722 (1974).
4. L. Landau andE. Lifchitz:Theorie du champ, sect.101 (Moscow, 1966).
5. L. Landau andE. Lifchitz:Theorie du champ, sect.94 (Moscow, 1966).