1. E. S. Fradkin, Zh. Éksp. Teor. Fiz.,29, 258 (1955).
2. E. S. Fradkin, Dokl. Akad. Nauk SSSR,98, 47;100, 879 (1954).
3. E. S. Fradkin, Dissertation [in Russian], Inst. Theor. Fiz. (1960) (Publ. in Trudy FIAN,29 (1965);
4. Acta Phys. Hung.,19, 176 (1965)).
5. E. S. Fradkin, Nucl. Phys.,76, 588 (1966).