On the Helly Number for Hyperplane Transversals to Unit Balls

We prove two results about the Hadwiger problem of finding the Helly number for line transversals of disjoint unit disks in the plane, and about its higher-dimensional generalization to hyperplane transversals of unit balls in d -dimensional Euclidean space. These consist of (a) a proof of the fact that the Helly number remains 5 even for arbitrarily large sets of disjoint unit disks—thus correcting a 40-year-old error; and (b) a lower bound of d+3 on the Helly number for hyperplane transversals to suitably separated families of unit balls in R ^d . Received January 25, 1999, and in revised form July 7, 1999.     

On the connected components of the space of line transversals to a family of convex sets

Let ℒ be the space of line transversals to a finite family of pairwise disjoint compact convex sets in ℝ³. We prove that each connected component of ℒ can itself be represented as the space of transversals to some finite family of pairwise disjoint compact convex sets. The research of J. E. Goodman was supported in part by NSF Grant DMS91-22065 and by NSA Grant MDA904-92-H-3069. R. Pollack's research was supported in part by NSF Grant CCR91-22103 and by NSA Grant MDA904-92-H-3075. The research of R. Wenger was supported in part by NSA Grant MDA904-93-H-3026 and by the NSF Regional Geometry Institute (Smith College, July 1993) Grant DMS90-13220.     

On the existence of partitionable skew Room frames

Partitionable skew Room frames of type hⁿ have played an important role in the constructions of 4-frames, (K₄-e)-frames and super-simple (4,2)-frames. In this paper, we investigate the existence of partitionable skew Room frames of type hⁿ. The necessary conditions for the existence of such a design are that and h?5. It is proved that these necessary conditions are also sufficient with a few possible exceptions. As a byproduct, the known results on the existence of skew Room frames and uniform 4-frames are both improved.     

Sharp Bounds on Geometric Permutations of Pairwise Disjoint Balls in R d

We prove that the maximum number of geometric permutations, induced by line transversals to a collection of n pairwise disjoint balls in \R ^d , is Θ (n ^d-1 ) . This improves substantially the upper bound of O(n ^2d-2 ) known for general convex sets [9]. We show that the maximum number of geometric permutations of a sufficiently large collection of pairwise disjoint unit disks in the plane is two, improving the previous upper bound of three given in [5]. Received September 21, 1998, and in revised form March 14, 1999.     

Counting disjoint 2-partitions for points in the plane

Disjoint partitions, and its counting, have been widely studied in the literature of optimal partitions and clustering. We give an exact counting on the number of disjoint ordered 2-partitions for n points in general position in R². We also give an exact counting on the maximum number of disjoint 2-partitions, where one part consists of two points, over all sets of n points in R².     

Line Transversals to Disjoint Balls

We prove that the set of directions of lines intersecting three disjoint balls in ℝ³ in a given order is a strictly convex subset of . We then generalize this result to n disjoint balls in ℝ ^d . As a consequence, we can improve upon several old and new results on line transversals to disjoint balls in arbitrary dimension, such as bounds on the number of connected components and Helly-type theorems.     

Geometric permutations and common transversals

The object of this paper is to study how many essentially different common transversals a family of convex sets on the plane can have. In particular, we consider the case where the family consists of pairwise disjoint translates of a single convex set.     

Partitionable sets,almost partitionable sets,and their applications

This paper introduces almost partitionable sets (APSs) to generalize the known concept of partitionable sets. These notions provide a unified frame to construct $\mathbb{Z}$‐cyclic patterned starter whist tournaments and cyclic balanced sampling plans excluding contiguous units. The existences of partitionable sets and APSs are investigated. As an application, a large number of optical orthogonal codes achieving the Johnson bound or the Johnson bound minus one are constructed.     

Lower bounds to helly numbers of line transversals to disjoint congruent balls

A line ℓ is a transversal to a family F of convex objects in ℝ ^d if it intersects every member of F. In this paper we show that for every integer d ⩾ 3 there exists a family of 2d−1 pairwise disjoint unit balls in ℝ ^d with the property that every subfamily of size 2d − 2 admits a transversal, yet any line misses at least one member of the family. This answers a question of Danzer from 1957. Crucial to the proof is the notion of a pinned transversal, which means an isolated point in the space of transversals. Here we investigate minimal pinning configurations and construct a family F of 2d−1 disjoint unit balls in ℝ ^d with the following properties: (i) The space of transversals to F is a single point and (ii) the space of transversals to any proper subfamily of F is a connected set with non-empty interior.     

A Constant Bound for Geometric Permutations of Disjoint Unit Balls

Abstract. We prove that a set of n disjoint unit balls in R ^d admits at most four distinct geometric permutations, or line transversals, thus settling a long-standing conjecture in combinatorial geometry. The constant bound significantly improves upon the Θ (n ^d-1 ) bound for disjoint balls of unrestricted radii.     

Latin squares with restricted transversals

We prove that for all odd m ≥ 3 there exists a latin square of order 3 m that contains an ( m ? 1 ) × m latin subrectangle consisting of entries not in any transversal. We prove that for all even n ≥ 10 there exists a latin square of order n in which there is at least one transversal, but all transversals coincide on a single entry. A corollary is a new proof of the existence of a latin square without an orthogonal mate, for all odd orders n ≥ 11 . Finally, we report on an extensive computational study of transversal‐free entries and sets of disjoint transversals in the latin squares of order n ? 9 . In particular, we count the number of species of each order that possess an orthogonal mate. © 2011 Wiley Periodicals, Inc. J Combin Designs 20:124‐141, 2012     

Existence of large sets of coverings with block size 3

Two types of large sets of coverings were introduced by T. Etzion (J Combin Designs, 2(1994), 359–374). What is maximum number (denoted by λ(n,k)) of disjoint optimal (n,k,k ? 1) coverings? What is the minimum number (denoted by µ(n,k)) of disjoint optimal (n,k,k ? 1) coverings for which the union covers the space? For k = 3, the numbers µ(n,k) have been determined with an unsolved order n = 17, and the numbers λ(n,k) have also been determined with an unsolved infinite class n ≡ 5 (mod 6). The unsolved numbers λ(n,3) and µ(17,3) will be completed in this note. This solution is based on the existence of a class of partitionable candelabra systems. © 2006 Wiley Periodicals, Inc. J Combin Designs 14: 400–405, 2006     

Strongly Signable and Partitionable Posets

The class ofStrongly Signablepartially ordered sets is introduced and studied. It is show that strong signability, reminiscent of Björner–Wachs' recursive coatom orderability, provides a useful and broad sufficient condition for a poset to be dual CR and hence partitionable. The flagh-vectors of strongly signable posets are therefore non-negative. It is proved that recursively shellable posets, polyhedral fans, and face lattices of partitionable simplicial complexes are all strongly signable, and it is conjectured that all spherical posets are. It is concluded that the barycentric subdivision of a partitionable complex is again partitionable, and an algorithm for producing a partitioning of the subdivision from a partitioning of the complex is described. An expression for the flagh-polynomial of a simplicial complex in terms of itsh-vector is given, and is used to demonstrate that the flagh-vector is symmetric or non-negative whenever theh-vector is.     

Allowable Interval Sequences and Line Transversals in the Plane

Given a family F of n pairwise disjoint compact convex sets in the plane with non-empty interiors, let T(k) denote the property that every subfamily of F of size k has a line transversal, and T the property that the entire family has a line transversal. We illustrate the applicability of allowable interval sequences to problems involving line transversals in the plane by proving two new results and generalizing three old ones. Two of the old results are Klee??s assertion that if F is totally separated then T(3) implies T, and the following variation of Hadwiger??s Transversal Theorem proved by Wenger and (independently) Tverberg: If F is ordered and each four sets of F have some transversal which respects the order on F, then there is a transversal for all of F which respects this order. The third old result (a consequence of an observation made by Kramer) and the first of the new results (which partially settles a 2008 conjecture of Eckhoff) deal with fractional transversals and share the following general form: If F has property T(k) and meets certain other conditions, then there exists a transversal of some m sets in F, with k<m<n. The second new result establishes a link between transversal properties and separation properties of certain families of convex sets.     

A Constant Bound for Geometric Permutations of Disjoint Unit Balls

   Abstract. We prove that a set of n disjoint unit balls in R ^d admits at most four distinct geometric permutations, or line transversals, thus settling a long-standing conjecture in combinatorial geometry. The constant bound significantly improves upon the Θ (n ^d-1 ) bound for disjoint balls of unrestricted radii.     

On the existence of room frames of type tu for u = 4 and 5

In this article, we describe some new constructions for Room frames and use them to show the existence of Room frames of types t⁴ and t⁵. Combining the result with those of Dinitz and Stinson leads to the following: the only cases for which the existence of Room frames is undecided are those types t⁴ where t = 14,22,26,34,38,46,62,74,82,86,98,122,134,146. © 1993 John Wiley & Sons, Inc.     

Large sets with holes

We study large sets of disjoint Steiner triple systems “with holes”. The purpose of these structures is to extend the use of large sets of disjoint Steiner triple systems in the construction of various other large set type structures to values of v for which no Steiner triple system of order v exists, i.e., v ≡ 0, 2, 4, or 5 (mod 6). We give constructions for all of these congruence classes. We describe several applications, including applications to large sets of disjoint group divisible designs and to large sets of disjoint separable ordered designs. © 1993 John Wiley & Sons, Inc.     

The existence of large set of symmetric partitioned incomplete latin squares

In this paper, we investigate the existence of large sets of symmetric partitioned incomplete latin squares of type g^u (LSSPILSs) which can be viewed as a generalization of the well‐known golf designs. Constructions for LSSPILSs are presented from some other large sets, such as golf designs, large sets of group divisible designs, and large sets of Room frames. We prove that there exists an LSSPILS(g^u) if and only if u ≥ 3, g(u ? 1) ≡ 0 (mod 2), and (g, u) ≠ (1, 5).     

Uniform room frames with five holes

In 1981, Dinitz and Stinson [2] proved that the necessary conditions were sufficient for the existence of a Room frame of type t^u for all u ≥ 6. Very recently, Zhu Lie and Ge Gennian [5] constructed all t⁵ Room frames except for four missing orders. In this article we construct t⁵ Room frames for t = 11,13,17, and 19; this completes the proof that the necessary conditions are sufficient for the existence of a Room frame of type t⁵. © 1993 John Wiley & Sons, Inc.     

On the number of minimal transversals in 3-uniform hypergraphs

We prove that the number of minimal transversals (and also the number of maximal independent sets) in a 3-uniform hypergraph with n vertices is at most cⁿ, where c≈1.6702. The best known lower bound for this number, due to Tomescu, is adⁿ, where d=10^1/5≈1.5849 and a is a constant.