1. A. Strumia:Nuovo Cimento B,101, 1 (1988).
2. A. Strumia:Nuovo Cimento B,101, 19 (1988).
3. Some results on the subject have been presented in form of communication inA. Strumia:Covariant Euler-Lagrange equations as systems of conservation laws with constrained field variables, in theIV Meeting Waves and Stability in Continuous Media, Taormina 1987, in press.
4. G. Boillat:C. R. Acad. Sci. Ser. A,283, 539 (1976).
5. G. Boillat:J. Math. Pures Appl.,56, 137 (1977).