Multiple transitivity except for a system of imprimitivity

TL;DR

This paper classifies finite block-faithful 2-by-block-transitive permutation group actions and proves the nonexistence of such actions for higher transitivity levels with nontrivial blocks.

Contribution

It provides a complete classification of finite block-faithful 2-by-block-transitive actions and establishes nonexistence results for higher levels of transitivity.

Findings

Classified all finite block-faithful 2-by-block-transitive actions.

Proved no finite block-faithful k-by-block-transitive actions exist for k ≥ 3 with nontrivial blocks.

Abstract

Let $\Omega$ be a set equipped with an equivalence relation $\sim$; we refer to the equivalence classes as blocks of $\Omega$. A permutation group $G \le \mathrm{Sym}(\Omega)$ is $k$-by-block-transitive if $\sim$ is $G$-invariant, with at least $k$ blocks, and $G$ is transitive on the set of $k$-tuples of points such that no two entries lie in the same block. The action is block-faithful if the action on the set of blocks is faithful. In this article we classify the finite block-faithful $2$-by-block-transitive actions. We also show that for $k \ge 3$, there are no finite block-faithful $k$-by-block-transitive actions with nontrivial blocks.