The extremal conjecture for S1S_1-free cyclic geometric hypergraphs

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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 n≥5n\geq 5, the conjectured extremal value is

The S1S_1 extremal conjecture.

ex⁡↻(n,S1)=Δ˙(n)+⌊n2⌋⌊n−22⌋.\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.

References

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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