Group divisible designs with two association classes

Abstract

We investigate group divisible designs with two association classes (known as GDDS, GADs or PBIBDs) with block size 3 and unequal size groups. We completely determine the necessary and sufficient conditions for groups with size vector (n, 1) for any n ≥ 3, and (n, 2, 1) for n ε {2, 3,..., 6}. We also have some general results for (n 1, n 2, n 3).