1. The proper-time parameter has been introduced byFock (1), and further used bySchwinger (2) andNambu (3). The relativistic classical mechanics based on the notion of proper time has been advocated byHorwitz andPiron (4). For the use of proper time in string models see,e.g., (5,6).
2. V. Fock:Žurn. Ėksp. Teor. Fiz.,12, 404 (1937).
3. Y. Nambu:Prog. Theor. Phys.,5, 82 (1950).
4. J. Schwinger:Phys. Rev.,82, 664 (1951).
5. L. P. Horwitz andC. Piron:Helv. Phys. Acta,46, 316 (1973).