1. Aarseth, S.J., Gott, J.R., and Turner, E.L.: 1979, Astrophys. J. 228, 664.
2. Abell, G.O.: 1958, Astrophys. J. Suppl. 3, 211.
3. Abell, G.O.: 1961, Astron. J. 66, 607.
4. Abell, G.O.: 1967, Astrophys. J. 72, 288.
5. Abell, G.O.: 1976, in A. R. Sandage, M. Sandage, and J. Kristian (eds.), Galaxies and the Universe, Chicago. Univ. Press, p. 601.