EKR conjecture for PSL(2,q) on pairs of projective-line points

Let qq be a prime power, and let PSL(2,q)\operatorname{PSL}(2,q) act on the 22-subsets of the projective line PG(1,q)\operatorname{PG}(1,q). Recall that the group has the Erdős–Ko–Rado (EKR) property when its intersection density is ρ(PSL(2,q))=1\rho(\operatorname{PSL}(2,q))=1. EKR conjecture. If q1(mod4)q\equiv 1\pmod 4, then ρ(PSL(2,q))=1\rho(\operatorname{PSL}(2,q))=1; if q3(mod4)q\equiv 3\pmod 4, then PSL(2,q)\operatorname{PSL}(2,q) does not have the EKR property. This prediction is based on computational results for the unresolved family of actions in line 13; the source does not state a resolution.

Sources & referencesView supporting material

Primary source

Andriaherimanana Sarobidy Razafimahatratra, “On the intersection density of primitive groups of degree a product of two odd primes”, arXiv:2109.05392 (2022).

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