Frankl's quadratic-threshold conjecture for critical intersecting hypergraphs

Let k>t≥1k>t\ge 1 be integers, let F\mathcal F be a kk-uniform, tt-intersecting hypergraph, and let τt(F)\tau_t(\mathcal F) be the minimum size of a set meeting every edge of F\mathcal F in at least tt vertices. Write d=k−td=k-t and define

m(k,t)=max⁡{∣F∣:F is t-intersecting and t-critical},m(k,t)=\max\{\lvert\mathcal F\rvert:\mathcal F\text{ is }t\text{-intersecting and }t\text{-critical}\},

where tt-critical means τt(F)=k\tau_t(\mathcal F)=k.

Frankl's conjecture. There exists a constant cc such that, whenever k>cd2k>c d^2,

m(k,k−d)=(k+dd).m(k,k-d)=\binom{k+d}{d}.

The complete kk-graph on a set of k+dk+d vertices attains the lower bound (k+dd)\binom{k+d}{d}. Frankl proved the equality for k≥d4k\ge d^4, and also proved the cases d=1d=1 and d=2d=2; the conjecture predicts a quadratic threshold in dd for the same extremal value.

References

Primary source

Lu Lu, Rongrong Lu, Qifan Wang and Tingzeng Wu, “An improved range for the maximum critically t-intersecting hypergraphs”, arXiv:2607.28253 (2026).

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