1. A. S. Davydov and A. A. Chaban, Nucl. Phys. 20, 499 (1960).
2. A. C. Davydov, Excited States of Nuclei (Atomizdat, Moscow, 1967) [in Russian].
3. A. C. Davydov, Sov. Phys. Usp. 8, 873 (1965).
4. A. Bohr, in Problems of Modern Physics, Collection of Articles (1955), No. 9, p. 9 [in Russian].
5. A. Bohr and B. R. Mottelson, Nuclear Structure, Vol. 2: Nuclear Deformations (World Scientific, Singapore, 1998).