Characterization of intersecting families of maximum size in PSL(2,q)
We consider the action of the 2-dimensional projective special linear group PSL(2,q) on the projective line PG(1,q) over the finite field Fq, where q is an odd prime power. A subset S of PSL(2,q) is said to be an intersecting family if for any g1,g2∈S, there exists an element x∈PG(1,q) such that xg1=xg2. It is known that the maximum size of an intersecting family in PSL(2,q) is q(q−1)/2. We prove that all intersecting families of maximum size are cosets of point stabilizers for all odd prime powers q>3.
Publication Source (Journal or Book title)
Journal of Combinatorial Theory. Series A
Long, L., Plaza, R., Sin, P., & Xiang, Q. (2018). Characterization of intersecting families of maximum size in PSL(2,q). Journal of Combinatorial Theory. Series A, 157, 461-499. https://doi.org/10.1016/j.jcta.2018.03.006