1. J. Schwinger, Phys. Rev.,73, 416 (1948).
2. R. G. Newton, Phys. Rev.,96, 523 (1954).
3. I. M. Ternov, V. G. Bagrov, V. A. Bordovitsyn, and O. F. Dorofeev, Dokl. Akad. Nauk SSSR,183, 810 (1968).
4. V. N. Baier, V. M. Katkov, and V. M. Strakhovenko, Yad. Fiz.,24, 379 (1976).
5. W. Pauli, Rev. Mod. Phys.,13, 203 (1941).