= Uk = 1/11 if fA < co and co if fA = 00. The theorem has important consequences in probability theory, and in 1951 de Bruijn and Erdos also used it to study recursion formulae.

As it happens, Erdos came very close to founding extremal graph theory before Tunin proved his theorem: in 1938, in connection with sequences of integers no one of which divided the product of two others, proved that for a quadrilateral C 4 we have ex(n; C 4 ) = O(n 3 / 2 ). However, at the time Erdos failed to see the significance of problems of this type: one of the very few occasions when Erdos was "blind". ) Erdos proved the following beautiful extension of Tunin's theorem (so the rest of the world had been blind).

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