1. Heitler, W.: Quantum Theory of Radiation, 3rd ed. London: Oxford University Press 1964
2. Apanasevich, P.A.: Optics and Spectroscopy16, 387 (1964)
3. Bergmann, S.M.: J. Math. Phys.8, 159 (1967)
4. Newstein, M.C.: Phys. Rev.167, 89 (1968)
5. Morozov, V.A.: Opt. Spectr.26, 62 (1969)