1. Shklovskii, I. S., Astron. Zhur 35, 557 (1958).
2. Lyttleton, R. A., The comets and their origin. (Cambridge University Press 1952 ).
3. Stephenson, C. B., Astrophys. J. 126, 195 (1957).
4. Redman, R. O., Observatory 79, 172 (1959).
5. Fesenkov, V. G., Astron. Zhur, 35, 327, 513 (1958).