[1] Katona, Nemetz, Simonowits:
On a problem of Turan in the theory of graphs. Mat. Lapok 15, 64, 228-238.
MR 0172263
[2] G. Ringel: On extremal problems in graph theory. in Proceedings of the Symp. held in Smolenice 1963, ČSAV Praha.
[3] J. Surányi: written communication to the author, 1973.