Let \(\mathcal{G}(n, m)\) denote the class of simple graphs on \(n\) vertices and \(m\) edges, and let \(G \in \mathcal{G}(n, m)\). For suitably restricted values of \(m\), \(G\) will necessarily contain certain prescribed subgraphs such as cycles of given lengths and complete graphs. For example, if \(m > \frac{1}{4}{n}^2\), then \(G\) contains cycles of all lengths up to \(\lfloor \frac{1}{2}(n+3) \rfloor\). Recently, we have established a number of results concerning the existence of certain subgraphs (cliques and cycles) in the subgraph of \(G\) induced by the vertices of \(G\) having some prescribed minimum degree. In this paper, we present some further results of this type. In particular, we prove that every \(G \in \mathcal{G}(n, m)\) contains a pair of adjacent vertices each having degree (in \(G\)) at least \(f(n, m)\) and determine the best possible value of \(f(n, m)\). For \(m > \frac{1}{4}{n}^{2}\), we find that \(G\) contains a triangle with a pair of vertices satisfying this same degree restriction. Some open problems are discussed.

A weighing matrix \(A = A(n, k)\) of order \(n\) and weight \(k\) is a square matrix of order \(n\), with entries \(0, \pm1\) which satisfies \(AA^T = kI_n\). H.C. Chan, C.A. Rodger, and J. Seberry “On inequivalent weighing matrices, \({Ars \; Combinatoria}\), \((1986) 21-A, 299-333\)” showed that there were exactly \(5\) inequivalent weighing matrices of order \(12\) and weight \(4\) and exactly \(2\) inequivalent matrices of weight \(5\). They showed that the weighing matrices of order \(12\) and weights \(2, 3\), and \(11\) were unique. Q.M. Husain “On the totality of the solutions for the symmetric block designs: \(\lambda = 2, k = 5\) or \(6\),” Sanky\(\bar{a}\) \(7 (1945), 204-208\)” had shown that the Hadamard matrix of order \(12\) (the weighing matrix of weight \(12\)) is unique. In this paper, we complete the classification of weighing matrices of order \(12\) by showing that there are seven inequivalent matrices of weight \(6\), three of weight \(7\), six of weight \(8\), four of weight \(9\), and four of weight \(10\). These results have considerable implications for inequivalence results for orders greater than 12.

Informally, a \((t, w, v; m)\)-threshold scheme is a way of distributing partial information (chosen from a set of \(v\) shadows) to \(w\) participants, so that any \(t\) of them can easily calculate one of \(m\) possible keys, but no subset of fewer than \(t\) participants can determine the key. A perfect threshold scheme is one in which no subset of fewer than \(t\) participants can determine any partial information regarding the key. In this paper, we study the number \(M(t, w, v)\), which denotes the maximum value of \(m\) such that a perfect \((t, w, v; m)\)-threshold scheme exists. It has been shown previously that\(M(t, w, v) \leq (v-t+1)/(w-t+1)\), with equality occurring if and only if there is a Steiner system \(S(t, w, v)\) that can be partitioned into Steiner systems \(S(t-1, w, v)\). In this paper, we study the numbers \(M(t, w, v)\) in some cases where this upper bound cannot be attained. Specifically, we determine improved bounds on the values \(M(3, 3, v)\) and \(M(4, 4, v)\).

A triple system \(B[3, \lambda; v]\) is indecomposable if it is not the union of two triple systems \(B[3, \lambda_1; v]\) and \(B[3, \lambda_2; v]\) with \(\lambda = \lambda_1 + \lambda_2\). We prove that indecomposable triple systems with \(\lambda = 6\) exist for \(v = 8, 14\) and for all \(v \geq 17\).

Given a convex lattice polygon with \(g\) interior lattice points, we find upper and lower bounds for the perimeter, diameter, and width of the polygon. For small \(g\), the extremal figures were found by computer.

We survey the existence of base sequences, that is, four sequences of lengths \(m+p, m+p, m, m, p\) odd with zero auto-correlation function, which can be used with Yang numbers and four disjoint complementary sequences (and matrices) with zero non-periodic (periodic) auto-correlation function to form longer sequences. We survey their application to make orthogonal designs \(OD(4t; t, t, t, t)\). We give the method of construction of \(OD(4t; t, t, t, t)\) for \(t = 1, 3, \ldots, 41, 45, \ldots, 65, 67\), \(69, 75, 77, 81, 85, 87, 91, 93, 95, 99, 101, 105\), \(111, 115, 117, 119, 123, 125, 129, 133, 141, \ldots, 147, 153\), \(155, 159, 161, 165, 169, 171, 175, 177, 183, 185, 189, 195, 201, 203, 205, 209\).

In this note we construct designs on the hexagonal grid to be used for choice experiments.

Digraph \(D\) is defined to be exclusive \((M, N)\)-transitive if, for each pair of vertices \(x\) and \(y\), for each \(xy\)-path \(P_1\) of length \(M\), there is an \(xy\)-path \(P_2\) of length \(N\) such that \(P_1 \cap P_2 = \{x, y\}\). It is proved that computation of a minimal edge augmentation to make \(K\) exclusive \((M, N)\)-transitive is NP-hard for \(M > N \geq 2\), even if \(D\) is acyclic. The corresponding decision problems are NP-complete. For \(N = 1\) and \(D = (V, E)\) with \(|V| = n\), an \(O(n^{M+3})\) algorithm to compute the exclusive \((M, 1)\)-transitive closure of an arbitrary digraph is provided.

Let \(v\), \(k\), and \(\lambda\) be positive integers. A perfect Mendelsohn design with parameters \(v\), \(k\), and \(\lambda\), denoted by \((v, k, \lambda)\)-PMD, is a decomposition of the complete directed multigraph \(\lambda K_v^*\) on \(v\) vertices into \(k\)-circuits such that for any \(r\), \(1 \leq r \leq k-1\), and for any two distinct vertices \(x\) and \(y\) there are exactly \(\lambda\) circuits along which the (directed) distance from \(x\) to \(y\) is \(r\). In this survey paper, we describe various known constructions, new results, and some further questions on PMDs.

A diagonal Latin square is a Latin square whose main diagonal and back diagonal are both transversals. It is proved in this paper that there are three pairwise orthogonal diagonal Latin squares of order \(n\) for all \(n \geq 7\) with \(28\) possible exceptions, in which \(118\) is the greatest one.

A graph \(G\) is \([a, b]\)-covered if each edge of \(G\) belongs to an \([a, b]\)-factor. Here, a necessary and sufficient condition for a graph to be \([a, b]\)-covered is given, and it is shown that an \([m, n]\)-graph is \([a, b]\)-covered if \(bm – na \geq 2(n-b)\) and \(0 \leq a < b \leq n\).