1. P. Havas, in Statistical Mechanics of Equilibrium and Non-Equilibrium, J. Meixner, ed. ( North-Holland, Amsterdam, 1965 ).
2. E. C. G. Stückelberg, Heiv. Phys. Acta 14, 372, 588 (1941)
3. E. C. G. Stückelberg, Heiv. Phys. Acta 15, 23 (1942).
4. L. P. Horwitz and C. Piron, Heiv. Phys. Acta 46, 316 (1973).
5. See J. R. Fanchi, Parameterized Relativistic Quantum Theory (Kluwer, Dordrecht, 1993), and references therein.