A Proof of the Cameron-Ku conjecture
Abstract
A family of permutations A ⊂ Sn is said to be intersecting if any two permutations in A agree at some point, i.e. for any σ, π ∈ A, there is some i such that σ(i)=π(i). Deza and Frankl showed that for such a family, |A| <= (n-1)!. Cameron and Ku showed that if equality holds then A = σ ∈ Sn: σ(i)=j for some i and j. They conjectured a `stability' version of this result, namely that there exists a constant c < 1 such that if A ⊂ Sn is an intersecting family of size at least c(n-1)!, then there exist i and j such that every permutation in A maps i to j (we call such a family `centred'). They also made the stronger `Hilton-Milner' type conjecture that for n ≥ 6, if A ⊂ Sn is a non-centred intersecting family, then A cannot be larger than the family C = σ ∈ Sn: σ(1)=1, σ(i)=i for some i > 2 (12), which has size (1-1/e+o(1))(n-1)!. We prove the stability conjecture, and also the Hilton-Milner type conjecture for n sufficiently large. Our proof makes use of the classical representation theory of Sn. One of our key tools will be an extremal result on cross-intersecting families of permutations, namely that for n ≥ 4, if A,B ⊂ Sn are cross-intersecting, then |A||B| ≤ ((n-1)!)2. This was a conjecture of Leader; it was recently proved for n sufficiently large by Friedgut, Pilpel and the author.