Consider \(n\) bridge teams each consisting of two pairs (the two pairs are called \({teammates}\)). A match is a triple \((i, j, b)\) where pair \(i\) opposes pair \(j\) on a board \(b\); here \(i\) and \(j\) are not teammates and “oppose” is an ordered relation. The problem is to schedule a tournament for the \(n\) teams satisfying the following conditions with a minimum number of boards:

1. Every pair must play against every other pair not on its team exactly once.
2. Every pair must play one match at every round.
3. Every pair must play every board exactly once except for odd \(n\), each pair can skip a board.
4. If pair \(i\) opposes pair \(j\) on a board, then the teammate of \(j\) must oppose the teammate of \(i\) on the same board.
5. Every board is played in at most one match at a round.

We call a schedule satisfying the above five conditions a \({complete \; coupling \; round \; robin \; schedule}\) (CCRRS) and one satisfying the first four conditions a \({coupling \; round \; robin \; schedule}\) (CRRS). While the construction of CCRRS is a difficult combinatorial problem, we construct CRRS for every \(n \geq 2\).

The concept of using basis reduction for finding \(t–(v,k,\lambda)\) designs without repeated blocks was introduced by D. L. Kreher and S. P. Radziszowski at the Seventeenth Southeastern International Conference on Combinatorics, Graph Theory and Computing. This tool and other algorithms were packaged into a system of programs that was called the design theory toolchest. It was distributed to several researchers at different institutions. This paper reports the many new open parameter situations that were settled using this toolchest.

We consider a generalization of the well-known gossip problem: Let the information of each point of a set \(X\) be conveyed to each point of a set \(Y\) by \(k\)-party conference calls. These calls are organized step-wisely, such that each point takes part in at most one call per step. During a call all the \(k\) participants exchange all the information they already know. We investigate the mutual dependence of the number \(L\) of calls and the number \(T\) of steps of such an information exchange. At first a general lower bound for \(L.k^T\) is proved. For the case that \(X\) and \(Y\) equal the set of all participants, we give lower bounds for \(L\) or \(T\), if \(T\) resp. \(L\) is as small as possible. Using these results the existence of information exchanges with minimum \(L\) and \(T\) is investigated. For \(k = 2\) we prove that for even \(n\), there is one of this kind if \(n \leq 8\).

Given a matrix in companion form over \(GF(2)\), whose characteristic polynomial is irreducible, a tridiagonal matrix, similar to the original one, is found, by constructing the similarity transformation. The theoretical basis is founded on the Lanczos tridiagonalization method, valid in the Complex domain. A variant of the Lanczos method, based on LU decomposition requirements, is modified to apply in the finite field \(GF(2)\). The work is derived from an application in VLSI design, where the matrices in companion form and in tridiagonal form represent two similar linear finite state machines, used for pseudo-random pattern generation and digital circuit testing. The construction of the similarity transformation between the matrices makes it possible to obtain directly the separate implementation of the two corresponding machines.

We determine all graphs \(G\) of order at least \(k + 1\), \(k \geq 3\), with the property that for any \(k\)-subset \(S\) of \(V(G)\) there is a unique vertex \(x, x \in V(G) – S\), which has exactly two neighbours in \(S\). Such graphs have exactly \(k + 1\) vertices and consist of a family of vertex-disjoint cycles. When \(k = 2\) it is clear that graphs with this property are the so-called friendship graphs.

In this paper, we deal with recursive constructions for incomplete group divisible designs (IGDDs). Denoting \(GD[k,1,v; uv] – GD[k,1,n; un]\) by \((u,k)-IGD[v,n]\), we will prove, as an application, that a \((7,4)-IGD[v,n]\) exists if and only if \(v \geq 3n\) and \(\text{v – n} \equiv 0 \pmod{2}\).

We investigate the labellings of sum graphs, necessary conditions for a graph to be a sum graph, and the range of edge numbers of sum graphs.

A set \(S\) of vertices of a graph is \(k\)-independent if each vertex in \(S\) is adjacent to at most \(k-1\) other vertices in \(S\). A graph \(G\) is well-\(k\)-covered if every maximal \(k\)-independent set is maximum. We shall characterize the well-\(k\)-covered trees and for \(k=2\) all such graphs of girth \(8\) or more.

We derive a first-order recurrence for \(a_n(t) = \sum_{k=0}^{n} \frac{(-1)^{n-k}}{1+tk} \binom{n}{k}\) (\(t\) fixed \(t\neq -\frac{1}{m}, m\in \mathbb{N}\)). The first-order recurrence yields an alternative proof for Riordan’s theorem: \(a_n(t) = \binom{1/{t+n}}{n}^{-1}(-1)^n\) and also yields the ordinary generating function \(\sum_{n=0}^{\infty} a_n(t) x^n\) for \(t \in \mathbb{N}.\)From the latter, one easily computes \(\sum_{n=0}^{\infty}a_n(t)\) which turns out to be the well-known \(\sum_{n=0}^{\infty} \frac{(-1)^n}{n+1} = \ln 2\) for \(t=1\). For \(t=2\), we get \(\sum_{n=0}^{\infty} (-1)^n\frac{2n}{(2n+1)} = \frac{\ln(\sqrt{2}+1)}{\sqrt{2}}\).

Avis has shown that the number of vertices of a minimal triangle-free \(5\)-chromatic graph is no fewer than \(19\). Mycielski has shown that this number is no more than \(23\). In this paper, we improve these bounds to \(21\) and \(22\), respectively.