1. Ambartsumian, V. A.: 1971, in D. J. K. O'Connell (ed.),Nuclei of Galaxies, North-Holland Publishing Co., Amsterdam, p. 9.
2. Arakelian, M. A., Dibay, E. A. and Yesipov, V. F.: 1975,Astrofiz. 11, 377.
3. Burbidge, G. R.: 1970,Ann. Rev. Astron. Astrophys. 8, 369.
4. Burbidge, G. R. and Burbidge, E. M.: 1967,Quasi-Stellar Objects, Freeman, San Francisco.
5. Burbidge, G. R., Crowne, A. H. and Smith, H. E.: 1977,Astrophys. J. Suppl. 33, 113.