Euclidean designs and coherent configurations

Abstract

The concept of spherical $t$-design, which is a finite subset of the unit sphere, was introduced by Delsarte-Goethals-Seidel (1977). The concept of Euclidean $t$-design, which is a two step generalization of spherical design in the sense that it is a finite weighted subset of Euclidean space, by Neumaier-Seidel (1988). We first review these two concepts, as well as the concept of tight $t$-design, i.e., the one whose cardinality reaches the natural lower bound. We are interested in $t$-designs (spherical or Euclidean) which are either tight or close to tight. As is well known by Delsarte-Goethals-Seidel (1977), in the study of spherical $t$-designs and in particular of those which are either tight or close to tight, association schemes play important roles. The main purpose of this paper is to show that in the study of Euclidean $t$-designs and in particular of those which are either tight or close to tight, coherent configurations play important roles. Here, coherent configuration is a purely combinatorial concept defined by D. G. Higman, and is obtained by axiomatizing the properties of general, not necessarily transitive, permutation groups, in the same way as association scheme was obtained by axiomatizing the properties of transitive permutation groups. The main purpose of this paper is to prove that Euclidean $t$-designs satisfying certain conditions give the structure of coherent configurations. Moreover we study the classification problems of Euclidean 4-designs on two concentric spheres with certain additional conditions.