Flag-transitive block designs and unitary groups

TL;DR

This paper classifies 2-designs with certain gcd conditions that admit flag-transitive automorphism groups, specifically those with socles isomorphic to projective special unitary groups, identifying all such designs within specific families or parameters.

Contribution

It provides a complete classification of 2-designs with gcd(r, λ)=1 admitting flag-transitive automorphism groups with socle PSU(n, q), including explicit parameter sets and families.

Findings

Designs belong to Hermitian unitals or Witt-Bose-Shrikhande spaces

Explicit parameter sets for exceptional designs

Complete classification of such designs with PSU(n, q) socle

Abstract

In this article, we study $2$-designs with $\gcd(r, \lambda)=1$ admitting a flag-transitive automorphism group. The automorphism groups of these designs are point-primitive of almost simple or affine type. We determine all pairs $(\mathcal{D}, G)$, where $\mathcal{D}$ is a $2$-design with $\gcd(r, \lambda)=1$ and $G$ is a flag-transitive almost simple automorphism group of $\mathcal{D}$ whose socle is $X=\mathrm{PSU}(n, q)$ with $(n, q)\neq (3, 2)$ and prove that such a design belongs to one of the two infinite families of Hermitian unitals and Witt-Bose-Shrikhande spaces, or it is isomorphic to a design with parameters $(6, 3, 2)$, $(7, 3, 1)$, $(8, 4, 3)$, $(10, 6, 5)$, $(11, 5, 2)$ or $(28, 7, 2)$.