Stability of Erdős–Ko–Rado theorems in circle geometries

Abstract

Circle geometries are incidence structures that capture the geometry of circles on spheres, cones and hyperboloids in three-dimensional space. In a previous paper, the author characterised the largest intersecting families in finite ovoidal circle geometries, except for Möbius planes of odd order. In this paper we show that also in these Möbius planes, if the order is greater than 3, the largest intersecting families are the sets of circles through a fixed point. We show the same result in the only known family of finite nonovoidal circle geometries. Using the same techniques, we show a stability result on large intersecting families in all ovoidal circle geometries. More specifically, we prove that an intersecting family $ℱ$ in one of the known finite circle geometries of order $q$, with $\mid ℱ\mid \ge \frac{1}{\sqrt{2}}{q}^2+2\sqrt{2}q+8$, must consist of circles through a common point, or through a common nucleus in case of a Laguerre plane of even order.