Analysing flag-transitive point-imprimitive 2-designs

Abstract

In this paper we develop several general methods for analysing flag-transitive point-imprimitive $2$-designs, which give restrictions on both the automorphisms and parameters of such designs. These constitute a tool-kit for analysing these designs and their groups. We apply these methods to complete the classification of flag-transitive, point-imprimitive $2$-$(v,k,\hat{I}»)$ designs with $\hat{I}»$ at most $4$.