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Tournament without 4-cycles

Source: 63 Polish MO 2012 Finals - Problem 4

April 23, 2018
combinatoricsTournament graphsgraph theoryPolandTournament

Problem Statement

nn players (n4n \ge 4) took part in the tournament. Each player played exactly one match with every other player, there were no draws. There was no four players (A,B,C,D)(A, B, C, D), such that AA won with BB, BB won with CC, CC won with DD and DD won with AA. Determine, depending on nn, maximum number of trios of players (A,B,C)(A, B, C), such that AA won with BB, BB won with CC and CC won with AA. (Attention: Trios (A,B,C)(A, B, C), (B,C,A)(B, C, A) and (C,A,B)(C, A, B) are the same trio.)