The conjectured fractional codegree threshold for generalized Steiner systems

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For integers n≥r≥2n\geq r\geq 2, a fractional (r−1,r,n)(r-1,r,n)-Steiner system in an rr-uniform hypergraph HH is a function φ:E(H)→R≥0\varphi:E(H)\to\mathbb{R}_{\geq0} such that deg⁡Hφ(p)=1\deg_H^\varphi(p)=1 for every (r−1)(r-1)-set p∈V(H)(r−1)p\in V(H)^{(r-1)}. Fractional Steiner-system threshold conjecture. The minimum codegree threshold for a fractional (r−1,r,n)(r-1,r,n)-Steiner system is

(r−1)nr+Θ(1).\frac{(r-1)n}{r}+\Theta(1).

This generalizes the paper's parity-based lower-bound construction and asks for the asymptotically sharp fractional codegree threshold; it remains open.

References

Primary source

Michael Zheng, “Codegree conditions for (fractional) Steiner triple systems”, arXiv:2411.07981 (2026).

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