Steiner Triple Systems Having a Prescribed Number of Triples in Common

Abstract

A Steiner triple system (briefly STS) is a pair where S is a finite set and is a collection of 3-subsets of S (called triples) such that every pair of distinct elements of S belongs to exactly one triple of . The number |S| is called the order of . It is well-known that there is an STS of order if and only if or 3 (mod 6). Therefore in saying that a certain property concerning STS is true for all it is understood that or 3 (mod 6). An STS of order v will sometimes be denoted by .