1. The main part of this investigation has been published in a series of articles byM. Sachs, inNuovo Cimento, since 1961. A set of review articles that summarize the results (until 1971) isa)Int. Journ. Theor. Phys.,4, 433 (1971);b)4, 453 (1971);c)5, 35 (1972);d)5, 161 (1972).
2. M. Sachs:Nuovo Cimento,53 B, 398 (1968).
3. M. Sachs:Int. Journ. Theor. Phys.,5, 35 (1972).
4. M. Sachs:Nuovo Cimento,47, 759 (1967).
5. M. Sachs:Nuovo Cimento,7 B, 247 (1972).