Steiner system
In combinatorial mathematics, a Steiner system (named after Jakob Steiner) is a type of block design, specifically a t-design with λ = 1 and t ≥ 2.
A Steiner system with parameters t, k, n, written S(t,k,n), is an n-element set S together with a set of k-element subsets of S (called blocks) with the property that each t-element subset of S is contained in exactly one block. In an alternate notation for block designs, an S(t,k,n) would be a t-(n,k,1) design.
This definition is relatively modern, generalizing the classical definition of Steiner systems which in addition required that k = t + 1. An S(2,3,n) was (and still is) called a Steiner triple (or triad) system, while an S(3,4,n) was called a Steiner quadruple system, and so on. With the generalization of the definition, this naming system is no longer strictly adhered to.
A long-standing problem in design theory was if any nontrivial (t < k < n) Steiner systems have t ≥ 6; also if infinitely many have t = 4 or 5. This was solved in the affirmative by Peter Keevash in 2014.
A finite projective plane of order q, with the lines as blocks, is an S(2, q + 1, q2 + q + 1), since it has q2 + q + 1 points, each line passes through q + 1 points, and each pair of distinct points lies on exactly one line.
A finite affine plane of order q, with the lines as blocks, is an S(2, q, q2). An affine plane of order q can be obtained from a projective plane of the same order by removing one block and all of the points in that block from the projective plane. Choosing different blocks to remove in this way can lead to non-isomorphic affine planes.
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