Small Group Divisible Steiner Quadruple Systems

A group divisible Steiner quadruple system, is a triple $(X, {\cal H}, {\cal B})$ where $X$ is a $v$-element set of points, ${\cal H} = \{H_1,H_2,\ldots,H_r\}$ is a partition of $X$ into holes and ${\cal B}$ is a collection of $4$-element subsets of $X$ called blocks such that every $3$-element subset is either in a block or a hole but not both. In this article we investigate the existence and non-existence of these designs. We settle all parameter situations on at most 24 points, with 6 exceptions. A uniform group divisible Steiner quadruple system is a system in which all the holes have equal size. These were called by Mills G-designs and their existence is completely settled in this article.

Blocking Sets in SQS(2v)

A Steiner quadruple system SQS(v) of order v is a family ℬ of 4-element subsets of a v-element set V such that each 3-element subset of V is contained in precisely one B ∈ ℬ. We prove that if T ∩ B ≠ ø for all B ∈ ℬ (i.e., if T is a transversal), then |T| ≥ v/2, and if T is a transversal of cardinality exactly v/2, then V \ T is a transversal as well (i.e., T is a blocking set). Also, in respect of the so-called ‘doubling construction’ that produces SQS(2v) from two copies of SQS(v), we give a necessary and sufficient condition for this operation to yield a Steiner quadruple system with blocking sets.