The extremal conjecture for S1S_1-free cyclic geometric hypergraphs

Let Ωn\Omega_n be the vertices of a regular nn-gon, and let ex(n,S1)\operatorname{ex}_\circlearrowright(n,S_1) denote the maximum number of triangles in an nn-vertex cyclic geometric hypergraph containing no copy of the configuration S1S_1. For n5n\geq 5, the conjectured extremal value is

The S1S_1 extremal conjecture.

ex(n,S1)=Δ˙(n)+n2n22.\operatorname{ex}_\circlearrowright(n,S_1)=\dot{\Delta}(n)+\left\lfloor\frac n2\right\rfloor\left\lfloor\frac{n-2}{2}\right\rfloor.

The authors state more broadly that all extremal S1S_1-free cyclic geometric hypergraphs should belong to the previously defined family H+(n)\mathcal{H}^+(n). The exact value is not determined in the paper; only lower and upper bounds are proved, so this conjecture remains open.

Sources & referencesView supporting material

Primary source

Zoltán Füredi, Dhruv Mubayi, Jason O'Neill and Jacques Verstraëte, “Extremal problems for pairs of triangles”, arXiv:2010.11100 (2020).

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