1. Springer-Verlag;WB Burton,1974
2. Cohen, R. S., Cong, H., Dame, T. M., and Thaddeus, P. 1980, Ap. J. (Letters) 239, L53.
3. Downes, D., Wilson, T. L., Bieging, J., and Wink, J. 1980, Astron. Astrophys. Suppl. 49, 379.
4. Elmegreen, B. 1979, Ap. J. 231, 372.
5. Georgelin, Y. M., and Georgelin, Y. P. 1976, Astron. Astrophys. 49, 57.