Turán’s Theorem for the Fano Plane


Confirming a conjecture of Vera T. Sós in a very strong sense, we give a complete solution to Turán's hypergraph problem for the Fano plane. That is we prove for n≥8 that among all 3-uniform hypergraphs on n vertices not containing the Fano plane there is indeed exactly one whose number of edges is maximal, namely the balanced, complete, bipartite hypergraph. Moreover, for n = 7 there is exactly one other extremal configuration with the same number of edges: the hypergraph arising from a clique of order 7 by removing all five edges containing a fixed pair of vertices.

For sufficiently large values n this was proved earlier by Füredi and Simonovits, and by Keevash and Sudakov, who utilised the stability method.

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We would like to thank Miklós Simonovits for sending us a copy of [21], Zoltán Füredi [6] for further information regarding the history of the problem, and the referees for a careful reading of this article.

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Correspondence to Christian Reiher.

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The second author was supported by the European Research Council (ERC grant PEPCo 724903).

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Bellmann, L., Reiher, C. Turán’s Theorem for the Fano Plane. Combinatorica 39, 961–982 (2019). https://doi.org/10.1007/s00493-019-3981-8

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Mathematics Subject Classification (2010)

  • 05C65
  • 05D05