Completely regular designs

We study a class of t-designs which enjoy a high degree of regularity. These are the subsets of vertices of the Johnson graph which are completely regular, in the sense of Delsarte [Philips Res. Reports Suppl. 10 (1973)]. After setting up the basic theory, we describe the known completely regular designs. We derive very strong restrictions which must hold in order for a design to be completely regular. As a result, we are able to determine which symmetric designs are completely regular and which Steiner systems with t = 2 are completely regular. © 1998 John Wiley & Sons, Inc. J Combin Designs 6: 261–273, 1998