Flag-transitive $4$-designs and $PSL(2,q)$ groups

Abstract

This paper considers flag-transitive $4$-$(q+1,k,\lambda)$ designs with $\lambda\geq5$ and $q+1>k>4$. Let the automorphism group of a design $\cal D$ be a simple group $G=PSL(2,q)$. Depend on the fact that the setwise stabilizer $G_B$ must be one of twelve kinds of subgroups, up to isomorphism we get the following two results. (i) If $10\geq \lambda \geq 5$, then except $(G,G_x,G_B,k,\lambda)=(PSL(2,761),{E_{761}}\rtimes {C_{380}},S_4,24,7)$ or $(PSL(2,512),{E_{512}}\rtimes {C_{511}},{D_{18}},18,8)$ undecided, $\cal D$ is a $4$-$(24,8,5)$, $4$-$(9,8,5)$, $4$-$(8,6,6)$, $4$-$(10,9,6)$, $4$-$(9,6,10)$, $4$-$(9,7,10)$, $4$-$(12,11,8)$ or $4$-$(14,13,10)$ design with $G_B=D_8$, ${E_8}\rtimes {C_7}$, $D_6$, ${E_9}\rtimes {C_4}$, $PSL(2,2)$, $D_{14}$, ${E_{11}}\rtimes {C_{5}}$ or ${E_{13}}\rtimes {C_6}$ respectively. (ii) If $\lambda>10$, ${G_B}=A_4$, $S_4$, $A_5$, $PGL(2,q_0)$($g>1$ even) or $PSL(2,q_0)$, where ${q_0}^g=q$, then there is no such design.