Diagonal groups and arcs over groups

TL;DR

This paper explores the structure of partitions and Latin squares related to diagonal groups over groups, extending previous work to more complex configurations and connecting them to arcs in projective spaces.

Contribution

It generalizes earlier results on Cartesian lattice partitions to cases with more partitions, investigates their group-theoretic properties, and links these structures to arcs in finite projective spaces.

Findings

For $m=2$, the structure reduces to mutually orthogonal Latin squares.

In the case $m>2$, the groups involved may all be isomorphic, but this is not proven.

Under certain conditions, the group $G$ must be abelian with specific automorphisms.

Abstract

In an earlier paper by three of the present authors and Csaba Schneider, it was shown that, for $m\ge2$, a set of $m+1$ partitions of a set $\Omega$, any $m$ of which are the minimal non-trivial elements of a Cartesian lattice, either form a Latin square (if $m=2$), or generate a join-semilattice of dimension $m$ associated with a diagonal group over a base group $G$. In this paper we investigate what happens if we have $m+r$ partitions with $r\geq 2$, any $m$ of which are minimal elements of a Cartesian lattice. If $m=2$, this is just a set of mutually orthogonal Latin squares. We consider the case where all these squares are isotopic to Cayley tables of groups, and give an example to show the groups need not be all isomorphic. For $m>2$, things are more restricted. Any $m+1$ of the partitions generate a join-semilattice admitting a diagonal group over a group $G$. It may be that the groups are all isomorphic, though we cannot prove this. Under an extra hypothesis, we show that $G$ must be abelian and must have three fixed-point-free automorphisms whose product is the identity. Under this hypothesis, such a structure gives an orthogonal array, and conversely in some cases. If the group is cyclic of prime order $p$, then the structure corresponds exactly to an arc of cardinality $m+r$ in the $(m-1)$-dimensional projective space over the field with $p$ elements, so all known results about arcs are applicable. More generally, arcs over a finite field of order $q$ give examples where $G$ is the elementary abelian group of order $q$. These examples can be lifted to non-elementary abelian groups using $p$-adic techniques.