Triples of involutions in PGL(2,q) and their incidence geometries

Abstract

For $q = p^n$ with $p$ an odd prime, the projective linear group $PGL(2,q)$ can be seen as the stabilizer of a conic $O$ in a projective plane $\pi = PG(2,q)$. In that setting, involutions of $PGL(2,q)$ correspond bijectively to points of $\pi$ not in $O$. Triples of involutions $\{ \alpha_P,\alpha_Q,\alpha_R \}$ of $PGL(2,q)$ can then be seen also as triples of points $\{P,Q,R\}$ of $\pi$. We investigate the interplay between algebraic properties of the group $H = \langle \alpha_P,\alpha_Q,\alpha_R \rangle$ generated by three involutions and geometric properties of the triple of points $\{P,Q,R\}$. In particular, we show that the coset geometry $\Gamma = \Gamma(H,(H_0,H_1,H_2))$, where $H_0 = \langle \alpha_Q,\alpha_R \rangle, H_1 = \langle \alpha_P,\alpha_R \rangle$ and $H_2 = \langle \alpha_P,\alpha_Q \rangle$ is a regular hypertope if and only if $\{P,Q,R\}$ is a strongly non self-polar triangle, a terminology we introduce. This entirely characterizes hypertopes of rank $3$ with automorphism group a subgroup of $PGL(2,q)$. As a corollary, we obtain the existence of hypertopes of rank $3$ with non linear diagrams and with automorphism group $PGL(2,q)$, for any $q = p^n$ with $p$ an odd prime. We also study in more details the case where the triangle $\{P,Q,R\}$ is tangent to $O$.