Since Moore digraphs do not exist for \( k \neq 1 \) and \( d \neq 1 \), the problem of finding digraphs of out-degree \( d \geq 2 \), diameter \( k \geq 2 \) and order close to the Moore bound becomes an interesting problem. To prove the non-existence of such digraphs or to assist in their construction (if they exist), we first may wish to establish some properties that such digraphs must possess. In this paper, we consider the diregularity of such digraphs. It is easy to show that any digraph with out-degree at most \( d \geq 2 \), diameter \( k \geq 2 \) and order one or two less than the Moore bound must have all vertices of out-degree \( d \). However, establishing the regularity or otherwise of the in-degree of such a digraph is not easy. In this paper, we prove that all digraphs of defect two are either diregular or almost diregular. Additionally, in the case of defect one, we present a new, simpler, and shorter proof that a digraph of defect one must be diregular, and in the case of defect two and for \( d = 2 \) and \( k \geq 3 \), we present an alternative proof that a digraph of defect two must be diregular.

In 1960, Hoffman and Singleton investigated the existence of Moore graphs of diameter 2 (graphs of maximum degree \(d\) and \(d^2 + 1\) vertices), and found that such graphs exist only for \(d = 2, 3, 7\) and possibly \(57\). In 1980, Erdős et al., using eigenvalue analysis, showed that, with the exception of \(C_4\), there are no graphs of diameter 2, maximum degree \(d\) and \(d^2\) vertices. In this paper, we show that graphs of diameter 2, maximum degree \(d\) and \(d^2 – 1\) vertices do not exist for most values of \(d\) with \(d \geq 6\), and conjecture that they do not exist for any \(d \geq 6\).

The distance \( d(u, v) \) between a pair of vertices \( u \) and \( v \) in a connected graph \( G \) is the length of a shortest path joining them. A vertex \( v \) of a connected graph \( G \) is an eccentric vertex of a vertex \( u \) if \( v \) is a vertex at greatest distance from \( u \); while \( v \) is an eccentric vertex of \( G \) if \( v \) is an eccentric vertex of some vertex of \( G \). A vertex \( v \) of \( G \) is a boundary vertex of a vertex \( u \) if \( d(u,w) \leq d(u,v) \) for each neighbour \( w \) of \( v \). A vertex \( v \) is a boundary vertex of \( G \) if \( v \) is a boundary vertex of some vertex of \( G \). It is easy to see that for a vertex \( u \), its eccentric vertices are boundary vertices for \( u \); but not conversely. In this paper, we introduce a new type of eccentricity called b-eccentricity and we study its properties.

A labeling of the vertices of a graph with distinct natural numbers induces a natural labeling of its edges: the label of an edge \((x,y)\) is the absolute value of the difference of the labels of \(x\) and \(y\). We say that a labeling of the vertices of a graph of order \(n\) is minimally \(k\)-equitable if the vertices are labeled with \(1, 2, \dots, n\) and in the induced labeling of its edges, every label either occurs exactly \(k\) times or does not occur at all. In this paper, we prove that Butterfly and Benes networks are minimally \(2^r\)-equitable where \(r\) is the dimension of the networks.

The method of large scale group testing has been used in the economical testing of blood samples, and in non-testing situations such as experimental designs and coding theory, for over \(50\) years. Some very basic questions addressing the minimum number of tests required to identify defective samples still remain unsolved, including the situation where one defective sample in each of two batches are to be found. This gives rise to an intriguing graph theoretical conjecture concerning bipartite graphs, a conjecture which in this paper is proved to be true in the case where vertices in one part of the bipartite graph have low degree.

Using the action of the linear fractional groups \( L_2(q) \), where \( q = 8, 25, 27, 29, 31 \) and \( 32 \), some \( 1 \)-designs are constructed. It is shown that subgroups of the automorphism group of \( L_2(q) \) appear as the full automorphism group of the constructed designs. In the cases \( q = 8 \) and \( 32 \), it is shown that the symmetric groups \( \mathbb{S}_9 \) and \( \mathbb{S}_{33} \), respectively, appear as the automorphism group of one of the constructed designs.

Here we consider string matching problems that arise naturally in applications to music retrieval. The \( \delta \)-Matching problem calculates, for a given text \( T_{1..n} \) and a pattern \( P_{1..m} \) on an alphabet of integers, the list of all indices \( \mathcal{I}_\delta = \{1 \leq i \leq n-m+1 : \max_{j=1}^m \{|P_j – T_{i+j-1}| \leq \delta\}\} \). The \( \gamma \)-Matching problem computes, for given \( T \) and \( P \), the list of all indices \( \mathcal{I}_\gamma = \{1 \leq i \leq n-m+1 : \sum_{j=1}^m |P_j – T_{i+j-1}| \leq \gamma\} \). In this paper, we extend the current result on the different matching problems to handle the presence of “\emph{don’t care}” symbols. We present efficient algorithms that calculate \( \mathcal{I}_\delta \), \( \mathcal{I}_\gamma \), and \( \mathcal{I}_{(\delta,\gamma)} = \mathcal{I}_\delta \cap \mathcal{I}_\gamma \) for pattern \( P \) with occurrences of “don’t cares”.

In 2004, Kim and Nakprasit showed that the chromatic number of \( K_2(9,4) \) is at least 11. In this note we present an 11-coloring for \( K^2(9,4) \) (the square of the Kneser graph \( K(9,4) \)). This proves that the chromatic number of \( K^2(9,4) \) is \(11\).

Let \( N \) and \( Z \) denote respectively the set of all nonnegative integers and the set of all integers. A \((p,q)\)-graph \( G = (V, E) \) is said to be additively \((a,r)\)-geometric if there exists an injective function \( f : V \to Z \) such that \( f^+(E) = \{a, ar, \dots, ar^{q-1}\} \) where \( a, r \in N \), \( r > 1 \), and \( f^+ \) is defined by \( f^+(uv) = f(u) + f(v) \) for all \( uv \in E \). If further \( f(v) \in N \) for all \( v \in V \), then \( G \) is said to be additively \((a,r)^*\)-geometric. In this paper we characterise graphs which are additively geometric and additively \(^*\)-geometric.

In this paper we find ten new weighing matrices of order \(2n\) and weight \(2n – 5\) constructed from two circulants, by forming a conjecture on the locations of the five zeros in a potential solution. Establishing patterns for the locations of zeros in sequences that can be used to construct weighing matrices seems to be a worthwhile path to explore, as it reduces significantly the computational complexity of the problem.