Encyclopaedia of DesignTheory: t-Designs
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Some non-degeneracy conditions are usually assumed, though there is no agreement about exactly what these should be. It is reasonable to assume that t<=k<=v. With this condition, if lambda=0, there are no blocks at all, so we usually assume that lambda>0. Now if t=k, then each set of k points occurs the same number of times as a block; while if k=v, then every block is incident with every point. These designs are not very interesting, so it is often assumed that t<k<v.
A t-design with lambda=1 is called a Steiner system. In particular, a 2-(v,3,1) design is a Steiner triple system or STS, and a 3-(v,4,1) design is a Steiner quadruple system or SQS. On the Steiner triple system page, you can see an example of a 2-(7,3,1) design.
A 2-design with k<v is also referred to as a balanced incomplete-block design or BIBD.
A t-design is said to have repeated blocks if there are two blocks incident with the same set of k points. A design with no repeated blocks is said to be simple. Thus, in a simple design, we can identify each block with the set of points incident with it.
However, the existence of simple t-designs in which not every k-set is a block is a much more difficult question, and was only settled affirmatively by Teirlinck in 1987. Teirlinck's designs have k = t+1, and the values of lambda and v are quite large.
One of the most important and difficult questions in design theory is:
For which values of t, k, v with t<k<v does a t-(v,k,1) Steiner system exist? In particular, do such designs exist for arbitrarily large values of t?At present, the largest value of t for which Steiner systems are known to exist is t=5.
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Peter J. Cameron
4 October 2004