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[2] Erdős P.:
Extremal problems in graph theory. Proceedings of the Symposium held in Smolenice in June 1963, Publ. H. Czechoslovak Acad. Sc., Prague 1964, 29-36.
MR 0180500
[3] Erdős P., Renyi A., Sós V. T.:
On a problem of graph theory. Studia Sci. Math, Hung. 1 (1966), 215-235.
MR 0223262