Discrete Mathematics and Optimization Seminar

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PETER KEEVASH
Princeton
Monday December 1st at 4.30pm
Burnside 1205



Title. The exact Turán number of the Fano plane.

Abstract. The Fano plane is the unique hypergraph with 7 triples on 7 vertices in which every pair of vertices is contained in a unique triple. Its edges correspond to
the lines of the projective plane over the field with two elements. The Turán problem is to find the maximum numebr of edges in a 3-uniform hypergraph
on n vertices not containing a Fano plane.

Noting that the Fano plane is not 2-colourable, but becomes so if one deletes an edge, a natural candidate is the largest 2-colourable 3-uniform hypergraph
on n vertices. This is obtained by partitioning the vertices into two parts, of sizes differing by at most one, and taking all the triples which intersect both
of them. Denote this hypergraph by H₂(n).

We show that for sufficiently large n, the unique largest 3-uniform hypergraph on n vertices not containing a Fano plane is H₂(n), thus proving a conjecture
of V. Sós raised in 1976. This is joint work with Benny Sudakov.