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2. W. Heitler: The Quantum Theory of Radiation, 3rd ed. (Oxford University Press, London 1954)
3. V.B. Berestetskii, E.M. Lifshitz, L.P. Pitaevskii: Relativistic Quantum Theory, P. 1 (Pergamon, Oxford 1971) § 63
4. L.D. Landau, E.M. Lifshitz: Quantum Mechanics: Non-Relativistic Theory, 3rd ed. (Pergamon, Oxford 1977) § 145
5. S. Rautian: In Invited Papers 2nd Conf. Interaction of Electrons with Strong Electromagnetic Fields (Central Research Institute for Physics, Budapest 1975)