1. G. Boillat:Lett. Nuovo Cimento,3, 521 (1970).
2. C. Cattaneo:Lincei. Rend. Sci. Fis. Mat. Nat.,46, 698 (1969).
3. f=1+i/c 2 is called « index » of the fluid byLichnerowicz (see ref. (2) in IV) and « relativistic enthalpy » byW. Israel:Journ. Math. Phys.,4, 1163 (1963).