Optimality and construction of some rectangular designs

Abstract

We obtain a sufficient condition forE-optimality of equireplicate designs. As an application, we proveE-optimality of certain types of three-class PBIBDs based on rectangular association scheme — in short — rectangular designs. These designs turn out to be highly efficient with respect to theA-criterion as well. We also observe that these designs, though themeselves not regular graph designs (RGD's) are yet strictlyE-better than every competing RGD, wheneverv≥26 andv=2 (mod 4). This provides an infinite series of counter examples to the conjecture of John and Mitchell (1977).

We also present two methods of construction of the rectangular designs. Apart from providing infinitely many examples of the designs provedE-optimal in this paper and in Cheng and Constantine (1986), this construction also provides — as a special case — the first known infinite series of most balanced group divisible designs, which were proved optimal with respect to all type 1 criteria by Cheng (1978).