Intersections among Steiner systems

A Steiner system S(l, m, n) is a system of subsets of size m (called blocks) from an n-set S, such that each d-subset from S is contained in precisely one block. Two Steiner systems have intersection k if they share exactly k blocks. The possible intersections among S(5, 6, 12)'s, among S(4, 5, 11)'s, among S(3, 4, 10)'s, and among S(2, 3, 9)'s are determined, together with associated orbits under the action of the automorphism group of an initial Steiner system. The following are results: (i) the maximal number of mutually disjoint S(5, 6, 12)'s is two and any two such pairs are isomorphic; (ii) the maximal number of mutually disjoint S(4, 5, 11)'s is two and any two such pairs are isomorphic; (iii) the maximal number of mutually disjoint S(3, 4, 10)'s is five and any two such sets of five are isomorphic; (iv) a result due to Bays in 1917 that there are exactly two non-isomorphic ways to partition all 3-subsets of a 9-set into seven mutually disjoint S(2, 3, 9)'s.     

Relative <Emphasis Type="Italic">t</Emphasis>-designs in binary Hamming association scheme <Emphasis Type="Italic">H</Emphasis>(<Emphasis Type="Italic">n</Emphasis>, 2)

A relative t-design in the binary Hamming association schemes H(n, 2) is equivalent to a weighted regular t-wise balanced design, i.e., certain combinatorial t-design which allows different sizes of blocks and a weight function on blocks. In this paper, we study relative t-designs in H(n, 2), putting emphasis on Fisher type inequalities and the existence of tight relative t-designs. We mostly consider relative t-designs on two shells. We prove that if the weight function is constant on each shell of a relative t-design on two shells then the subset in each shell must be a combinatorial \((t-1)\)-design. This is a generalization of the result of Kageyama who proved this under the stronger assumption that the weight function is constant on the whole block set. Using this, we define tight relative t-designs for odd t, and a strong restriction on the possible parameters of tight relative t-designs in H(n, 2). We obtain a new family of such tight relative t-designs, which were unnoticed before. We will give a list of feasible parameters of such relative 3-designs with \(n \le 100\), and then we discuss the existence and/or the non-existence of such tight relative 3-designs. We also discuss feasible parameters of tight relative 4-designs on two shells in H(n, 2) with \(n \le 50\). In this study we come up with the connection on the topics of classical design theory, such as symmetric 2-designs (in particular 2-\((4u-1,2u-1,u-1)\) Hadamard designs) and Driessen’s result on the non-existence of certain 3-designs. We believe Problems 1 and 2 presented in Sect. 5.2 open a new way to study relative t-designs in H(n, 2). We conclude our paper listing several open problems.     

A construction of mutually disjoint Steiner systems from isomorphic Golay codes

It is well known that the extended binary Golay [24,12,8] code yields 5-designs. In particular, the supports of all the weight 8 codewords in the code form a Steiner system S(5,8,24). In this paper, we give a construction of mutually disjoint Steiner systems S(5,8,24) by constructing isomorphic Golay codes. As a consequence, we show that there exists at least 22 mutually disjoint Steiner systems S(5,8,24). Finally, we prove that there exists at least 46 mutually disjoint 5-(48,12,8) designs from the extended binary quadratic residue [48,24,12] code.     

Partitioning the planes of AG2m(2) into 2-designs

A t-design S_λ(t, k, v) is an arrangement of v elements in blocks of k elements each such that every t element subset is contained in exactly λ blocks. A t-design S_λ(t, k, v) is called t′-resolvable if the blocks can be partitioned into families such that each family is the block system of a S_λ(t′, k, v). It is shown that the S₁(3, 4, 2^2m) design of planes on an even dimensional affine space over the field of two elements is 2-resolvable. Each S₁(2, 4, 2^2m) given by the resolution is itself 1-resolvable. As a corollary it is shown that every odd dimensional projective space over the field of two elements admits a 1-packing of 1-spreads, i.e. a partition of its lines into families of mutually disjoint lines whose union covers the space. This 1-packing may be generated from any one of its spreads by repeated application of a fixed collineation.     

Halving Steiner 2-designs

A Steiner 2-design S(2,k,v) is said to be halvable if the block set can be partitioned into two isomorphic sets. This is equivalent to an edge-disjoint decomposition of a self-complementary graph G on v vertices into K_ks. The obvious necessary condition of those orders v for which there exists a halvable S(2,k,v) is that v admits the existence of an S(2,k,v) with an even number of blocks. In this paper, we give an asymptotic solution for various block sizes. We prove that for any k?5 or any Mersenne prime k, there is a constant number v₀ such that if v>v₀ and v satisfies the above necessary condition, then there exists a halvable S(2,k,v). We also show that a halvable S(2,2ⁿ,v) exists for over a half of possible orders. Some recursive constructions generating infinitely many new halvable Steiner 2-designs are also presented.     

An analogue of group divisible designs for Moore graphs

We study graphs whose adjacency matrix S of order n satisfies the equation S + S² = J ? K + kI, where J is a matrix of order n of all 1's, K is the direct sum on $\text{n}\text{l}$ matrices of order l of all 1's, and I is the identity matrix. Moore graphs are the only solutions to the equation in the case l = 1 for which K = I. In the case k = l we can obtain Moore graphs from a solution S by a bordering process analogous to obtaining (ν, κ, λ)-designs from some group divisible designs. Other parameters are rare. We are able to find one new interesting graph with parameters k = 6, l = 4 on n = 40 vertices. We show that it has a transitive automorphism group isomorphic to C₄ × S₅.     

Some resolutions of S(5, 8, 24)

There exist 13 mutually disjoint resolutions of the Steiner system S(5, 8, 24). There also exist nine nonisomorphic mutually disjoint resolutions of S(5, 8, 24) where three of the resolutions have the same L₂(23) as an automorphism group and the other six have the same affine group C₂₃¹¹ as an automorphism group. A resolution of S(5, 8, 24) using a group of order 21 is displayed and a 13-dimensional Room-type design is mentioned.     

Constructions of Spherical 3-Designs

 Spherical t-designs are Chebyshev-type averaging sets on the d-sphere which are exact for polynomials of degree at most t. This concept was introduced in 1977 by Delsarte, Goethals, and Seidel, who also found the minimum possible size of such designs, in particular, that the number of points in a 3-design on S ^d must be at least . In this paper we give explicit constructions for spherical 3-designs on S ^d consisting of n points for d=1 and ; d=2 and ; d=3 and ; d=4 and ; and odd or even. We also provide some evidence that 3-designs of other sizes do not exist. We will introduce and apply a concept from additive number theory generalizing the classical Sidon-sequences. Namely, we study sets of integers S for which the congruence mod n, where and , only holds in the trivial cases. We call such sets Sidon-type sets of strength t, and denote their maximum cardinality by s(n, t). We find a lower bound for s(n, 3), and show how Sidon-type sets of strength 3 can be used to construct spherical 3-designs. We also conjecture that our lower bound gives the true value of s(n, 3) (this has been verified for n≤125). Received: June 19, 1996     

A Note on Partial Parallel Classes in Steiner Systems

A partial parallel class of blocks of a Steiner system S(t,k,v) is a collection of pairwise disjoint blocks. The purpose of this note is to show that any S(k,k+1,v) Steiner system, with v?k⁴+3k³+k²+1, has a partial parallel class containing at least (v?k+1)/(k+2) blocks.     

Eine charakterisierung des steinersystems S(5, 8, 24)

S(5, 8, 24) is characterized as the unique Steiner system S(t, k, n) satisfying n = (t + 1)(k ? t + 1) and k ? t + 2 ? 4.     

Indecomposable triple systems

A t-design (λ, t, d, n) is a system $\text{B}$ of sets of size d from an n-set S, such that each t subset of S is contained in exactly λ elements of $\text{B}$. A t-design is indecomposable (written IND(λ, t, d, n)) if there does not exist a subset $\text{B}$ ? $\text{B}$ such that $\text{B}$ is a (λ, t, d, n) for some λ, 1 ? λ < λ. A triple system is a (λ; 2, 3, n). Recursive and constructive methods (several due to Hanani) are employed to show that: (1) an IND(2; 2, 3, n) exists for n ≡ 0, 1 (mod 3), n ? 4 and n ≡ 7 (designs of Bhattacharya are used here), (2) an IND(3; 2, 3, n) exists for n odd, n ? 5, (3) if an IND(λ, 2, 3, n) exists, n odd, then there exists an infinite number of indecomposable triple systems with that λ.     

On antipodal Euclidean tight (2e + 1)-designs

Neumaier and Seidel (1988) generalized the concept of spherical designs and defined Euclidean designs in ℝ ⁿ . For an integer t, a finite subset X of ℝ ⁿ given together with a weight function w is a Euclidean t-design if holds for any polynomial f(x) of deg(f)≤ t, where {S _i , 1≤ i ≤ p} is the set of all the concentric spheres centered at the origin that intersect with X, X _i = X∩ S _i , and w:X→ ℝ_{> 0}. (The case of X⊂ S ⁿ⁻¹ with w≡ 1 on X corresponds to a spherical t-design.) In this paper we study antipodal Euclidean (2e+1)-designs. We give some new examples of antipodal Euclidean tight 5-designs. We also give the classification of all antipodal Euclidean tight 3-designs, the classification of antipodal Euclidean tight 5-designs supported by 2 concentric spheres.     

On λ-designs

A λ-design is a system of subsets S₁, S₂,…, S_n from an n-set S, n > 3, where |S_i ∩ S_j| = λ for i ≠ j, |S_j| = k_j > λ > 0, and not all k_j, are equal. Ryser [9] and Woodall [101 have shown that each element of S occurs either r₁, or r₂ times (r₁ ≠ r₂) among the sets S₁,…, S_n and r₁ +r₂ = n + 1. Here we: (i) mention most of what is currently known about λ-designs; (ii) provide simpler proofs of some known results; (iii) present several new general theorems; and (iv) apply our theorems and techniques to the calculation of all λ-designs for λ ? 5. In fact, this calculation has been done for all λ ?/ 9 and is available from the author.     

A note on the probabilistic approach to Turán's problem

The Turán number T(n, l, k) is the smallest possible number of edges in a k-graph on n vertices such that every l-set of vertices contains an edge. Given a k-graph H = (V(H), E(H)), we let X_s(S) equal the number of edges contained in S, for any s-set S?V(H). Turán's problem is equivalent to estimating the expectation E(X_l), given that min(X_l) ≥ 1. The following lower bound on the variance of X_s is proved:

$\text{Var(X}{}_{\mathrm{s}}\text{)?mm}\text{n?2k}\text{s?k}\text{n}\text{s}{}^{\mathrm{?1}}\text{n}\text{k}{}^1$

, where m = |E(H)| and $\text{m}\text{= (}{}_{\mathrm{k}}{}^{\mathrm{n}}\text{) ? m}$. This implies the following: putting t(k, l) = lim_n→∞T(n, l, k)(_kⁿ)^?1 then t(k, l) ≥ T(s, l, k)((_k^s) ? 1)^?1, whenever s ≥ l > k ≥ 2. A connection of these results with the existence of certain t-designs is mentioned.     

A census of highly symmetric combinatorial designs

As a consequence of the classification of the finite simple groups, it has been possible in recent years to characterize Steiner t-designs, that is t-(v,k,1) designs, mainly for t=2, admitting groups of automorphisms with sufficiently strong symmetry properties. However, despite the finite simple group classification, for Steiner t-designs with t>2 most of these characterizations have remained long-standing challenging problems. Especially, the determination of all flag-transitive Steiner t-designs with 3≤t≤6 is of particular interest and has been open for about 40 years (cf. Delandtsheer (Geom. Dedicata 41, p. 147, 1992 and Handbook of Incidence Geometry, Elsevier Science, Amsterdam, 1995, p. 273), but presumably dating back to 1965). The present paper continues the author’s work (see Huber (J. Comb. Theory Ser. A 94, 180–190, 2001; Adv. Geom. 5, 195–221, 2005; J. Algebr. Comb., 2007, to appear)) of classifying all flag-transitive Steiner 3-designs and 4-designs. We give a complete classification of all flag-transitive Steiner 5-designs and prove furthermore that there are no non-trivial flag-transitive Steiner 6-designs. Both results rely on the classification of the finite 3-homogeneous permutation groups. Moreover, we survey some of the most general results on highly symmetric Steiner t-designs.      

t12-Designs

Every (t + 1)-design $\text{B}$ satisfies (+) If T is a set of t points, and B a block of $\text{B}$ then the number α(T, B) of flags (x, A) with x?T, x?B, T ∪ {x} ? A depends only on |T ∩ B|.A t-design with property (+) is called a $\text{t}\text{1}\text{2}\text{-}\text{design}$. The most interesting general classes of t-designs are $\text{t}\text{1}\text{2}\text{-}\text{designs}$: Hadamard 3-designs are $\text{3}\text{1}\text{2}\text{-}\text{designs}$, symmetric 2-designs are $\text{2}\text{1}\text{2}\text{-}\text{designs}$, and dual 2-designs, transversal designs, and partial geometries are $\text{1}\text{1}\text{2}\text{-}\text{designs}$; in fact, $\text{1}\text{1}\text{2}\text{-}\text{designs}$ share most properties of partial geometries.$\text{1}\text{1}\text{2}\text{-}\text{designs}$ are studied in detail, and their connection with strongly regular graphs is investigated.It is shown that $\text{t}\text{1}\text{2}\text{-}\text{designs}$ behave like t-designs with respect to derivation, residuals, and complementation.Various characterizations of partial geometries, generalized quadrangles, symmetric 2-designs, and Hadamard 3-designs are given in terms of $\text{t}\text{1}\text{2}\text{-}\text{designs}$.The paper ends with a proof that $\text{t}\text{1}\text{2}\text{-}\text{designs}$ with t ? 4 are already (t + 1)-designs.     

New 5-designs with automorphism group PSL(2, 23)

Blocks of the unique Steiner system S(5, 8, 24) are called octads. The group PSL(2, 23) acts as an automorphism group of this Steiner system, permuting octads transitively. Inspired by the discovery of a 5-(24, 10, 36) design by Gulliver and Harada, we enumerate all 4- and 5-designs whose set of blocks are union of PSL(2, 23)-orbits on 10-subsets containing an octad. © 1999 John Wiley & Sons, Inc. J Combin Designs 7: 147–155, 1999     

On the Number of Blocks in a Generalized Steiner System

We considert-designs withλ=1 (generalized Steiner systems) for which the block size is not necessarily constant. An inequality for the number of blocks is derived. Fort=2, this inequality is the well known De Bruijn–Erd?s inequality. Fort>2 it has the same order of magnitude as the Wilson–Petrenjuk inequality for Steiner systems with constant block size. The point of this note is that the inequality is very easy to derive and does not seem to be known. A stronger inequality was derived in 1969 by Woodall (J. London Math. Soc.(2)1, 509–519), but it requires Lagrange multipliers in the proof.     

Some Indecomposable t-Designs

The existence of large sets of 5-(14,6,3) designs is in doubt. There are five simple 5-(14,6,6) designs known in the literature. In this note, by the use of a computer program, we show that all of these designs are indecomposable and therefore they do not lead to large sets of 5-(14,6,3) designs. Moreover, they provide the first counterexamples for a conjecture on disjoint t-designs which states that if there exists a t-(v, k, λ) design (X, D) with minimum possible value of λ, then there must be a t-(v, k, λ) design (X, D′) such that D∩ D′ = Ø.     

On the structure of 1-designs with at most two block intersection numbers

We introduce the notion of an unrefinable decomposition of a 1-design with at most two block intersection numbers, which is a certain decomposition of the 1-designs collection of blocks into other 1-designs. We discover an infinite family of 1-designs with at most two block intersection numbers that each have a unique unrefinable decomposition, and we give a polynomial-time algorithm to compute an unrefinable decomposition for each such design from the family. Combinatorial designs from this family include: finite projective planes of order n; SOMAs, and more generally, partial linear spaces of order (s, t) on (s + 1)² points; as well as affine designs, and more generally, strongly resolvable designs with no repeated blocks.