Block-transitive designs with a poset of imprimitive partitions

Abstract We study block designs which admit an automorphism group that is transitive on blocks and points, and leaves invariant every partition in a given finite poset of partitions of the point set. The full stabiliser G of all the partitions in the poset is a generalised wreath product. We use the theory of generalised wreath products to give necessary and sufficient conditions, in terms of the ‘array’ of a point-subset B , for the set of G -images of B to form the block-set of a G -block-transitive 2-design. This generalises previous results for the special cases where the poset is a chain or an anti-chain. We also give explicit infinite families of examples of 2-designs for each poset involving three proper partitions, and for the famous N -poset with four partitions. (Posets with two proper partitions have been treated previously.) This suggests the problem of finding explicit examples for other posets.