IMO 1979 LL BEL2

For a finite set E of cardinality n \geq3, let f(n) denote the

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IMO 1979 LL BEL2

Origin: BEL

Problem

For a finite set E of cardinality n \geq3, let f(n) denote the maximum number of 3-element subsets of E, any two of them having exactly one common element. Calculate f(n).