1. L.D. Landau, J. Phys. USSR 11, 91 (1947)
2. L.D. Landau, Phys. Rev. 75, 884 (1949)
3. E. Andronikashvili, Zh. Esksp. Theor. Fiz. 16, 780 (1946)
4. E. Andronikashvili, Zh. Esksp. Theor. Fiz. 18, 424 (1948)
5. R.P. Feynman, Phys. Rev. 94, 262 (1954)