1. V. I. MAN'KO, ‘Quantum Electrodynamics with External Field’ (Tomsk University, Tomsk, 1978) p. 101.
2. S. G. KRIVOSHLYKOV and I. N. SISSAKIAN,Opt. Quantum Electron. 12 (1980) 463.
3. S. G. KRIVOSHLYKOV, N. I. PETROV and I. N. SISSAKIAN, ‘Group Theoretical Methods in Physics’; Proceedings of the International 1982 Seminar (Nauka, Moscow, 1983) p. 235.
4. Idem, Pis'ma Zh. Tech. Fiz. 9 (1983) 1489.
5. V. FANO,Rev. Mod. Phys. 29 (1957) 74.