1. V N Levkovskii Soviet Phys. JETP 4 291 (1957)
2. V N Levkovskii Soviet Phys. JETP 6 1174 (1958)
3. V N Levkovskii Soviet Phys. JETP 18 213 (1964)
4. V N Levkovskii Soviet Journal of Nuclear Physics 18 705 (1973)
5. D G Gardner Nucl. Phys. 29 373 (1962)