Higher-dimensional grid-imprimitive block-transitive designs

Abstract It was shown in 1989 by Delandtsheer and Doyen that, for a 2-design with v points and block size k , a block-transitive group of automorphisms can be point-imprimitive (that is, leave invariant a nontrivial partition of the point set) only if v is small enough relative to k . Recently, exploiting a construction of block-transitive point-imprimitive 2-designs given by Cameron and the last author, four of the authors studied 2-designs admitting a block-transitive group that preserves a two-dimensional grid structure on the point set. Here we consider the case where there a block-transitive group preserves a multidimensional grid structure on points. We provide necessary and sufficient conditions for such 2-designs to exist in terms of the parameters of the grid, and certain ‘array parameters’ which describe a subset of points (which will be a block of the design). Using this criterion, we construct explicit examples of 2-designs for grids of dimensions three and four, and pose several open questions.