Modifications of the "central-method" to construct Steiner triple systems

Let V with IV1 = v be a finite set and B a set of 3-subsets of V. The elements of V are called points, those of B lines. If any 2-subset of V is contained in exactly one line, then the pair (V, B) is called a Steiner triple system of order V, in short STS(u). Each point lies on exactly r = i(v 1) lines and we have JB1 = b = &(v 1). The condition v = 7, 9 + 6n, IZ E No, is necessary and sufficient for the existence of STS(u) (the trivial cases u = 1, u = 3 are excluded). The set of these “admissible” numbers, of these “Steiner numbers” is denoted by STS.