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