1. V. I. Fushchich and A. G. Nikitin, Symmetry of Maxwell's Equations [in Russian], Naukova Dumka, Kiev (1983).
2. V. S. Vladimirov and I. V. Volovich, Usp. Mat. Nauk,40, 17 (1985).
3. V. S. Vladimirov and I. V. Volovich, Dokl. Akad. Nauk SSSR,280, 328 (1985).
4. L. Abellanas and A. Galindo, Commun. Math. Phys.,79, 341 (1981).
5. S. Hojman, J. Phys. A,17, 2399 (1984).