English

Characterization of intersecting families of maximum size in $PSL(2,q)$

Combinatorics 2020-01-30 v3

Abstract

We consider the action of the 22-dimensional projective special linear group PSL(2,q)PSL(2,q) on the projective line PG(1,q)PG(1,q) over the finite field \Fq\F_q, where qq is an odd prime power. A subset SS of PSL(2,q)PSL(2,q) is said to be an intersecting family if for any g1,g2Sg_1,g_2 \in S, there exists an element xPG(1,q)x\in PG(1,q) such that xg1=xg2x^{g_1}= x^{g_2}. It is known that the maximum size of an intersecting family in PSL(2,q)PSL(2,q) is q(q1)/2q(q-1)/2. We prove that all intersecting families of maximum size are cosets of point stabilizers for all odd prime powers q>3q>3.

Keywords

Cite

@article{arxiv.1608.07304,
  title  = {Characterization of intersecting families of maximum size in $PSL(2,q)$},
  author = {Ling Long and Rafael Plaza and Peter Sin and Qing Xiang},
  journal= {arXiv preprint arXiv:1608.07304},
  year   = {2020}
}

Comments

32 pages, corrected some minor errors; in particular, stated the new Theorem 1 and 3 with the extra condition that $q>3$