Intersection density conjectures for transitive permutation groups

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Let G≤Sym⁡(Ω)G\leq \operatorname{Sym}(\Omega) be a transitive permutation group. For ω∈Ω\omega\in\Omega, write Gω={g∈G:ωg=ω}G_\omega=\{g\in G:\omega^g=\omega\} and define the intersection density of GG by

ρ(G)=1∣Gω∣max⁡{∣F∣:F⊆G is intersecting},\rho(G)=\frac{1}{|G_\omega|}\max\{|\mathcal F|:\mathcal F\subseteq G\text{ is intersecting}\},

where F\mathcal F is intersecting if for every g,h∈Fg,h\in\mathcal F there exists ω∈Ω\omega\in\Omega such that ωg=ωh\omega^g=\omega^h.

Intersection density conjectures. (i) If ∣Ω∣|\Omega| is a prime power, then ρ(G)=1\rho(G)=1. (ii) If ∣Ω∣=2p|\Omega|=2p, where pp is an odd prime, then 1≤ρ(G)≤21\leq\rho(G)\leq 2. (iii) If ∣Ω∣=pq|\Omega|=pq, where pp and qq are two odd primes, then ρ(G)=1\rho(G)=1.

These conjectures extend the Erdős–Ko–Rado theorem to transitive permutation groups and seek sharp restrictions on the size of intersecting families. The third assertion was disproved by constructing transitive groups of degree pqpq, with p=(qr−1)/(q−1)p=(q^r-1)/(q-1), whose intersection density is qq; consequently, the combined conjectural statement is refuted.

References

Primary source

Roghayeh Maleki and Andriaherimanana Sarobidy Razafimahatratra, “The Erdős-Ko-Rado Theorem for non-quasiprimitive groups of degree 3p”, arXiv:2309.09906 (2023).

Additional references

2 papers in this index state this conjecture (2021–2023). The statement above is taken from the most recent of them; the others are arXiv:2109.05392.

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