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 \(0 \leq a < b \leq n\).

The chromatic polynomial captures a good deal of combinatorial information about a graph, describing its acyclic orientations, its all-terminal reliability, its spanning trees, as well as its colorings. Several methods for computing the chromatic polynomial of a graph G construct a computation tree for G whose leaves are “simple” base graphs for which the chromatic polynomial is readily found. Previously studied methods involved base graphs which are complete graphs, completely disconnected graphs, forests, and trees. In this paper, we consider chordal graphs as base graphs. Algorithms for computing the chromatic polynomial based on these concepts are developed, and computational results are presented.

Using several computer algorithms, we calculate some values and bounds for the function \(e(3,k,n)\), the minimum number of edges in a triangle-free graph on \(n\) vertices with no independent set of size \(k\). As a consequence, the following new upper bounds for the classical two-color Ramsey numbers are obtained:\(R(3,10) \leq 43\), \(\quad\),\(R(3,11) \leq 51\), \(\quad\),\(R(3,12) \leq 60\), \(\quad\),\(R(3,13) \leq 69\) \(\quad\) and,\(R(3,14) \leq 78\).

We give some results on the excess of Hadamard matrices. We provide a list for Hadamard matrices of order \(\leq 1000\) of the smallest upper bounds known for the excess for each order. A construction is indicated for the maximal known excess.

The type of a \(3\)-factorization of \(3K_{2n}\) is the pair \((t,s)\), where \(t\) is the number of doubly repeated edges in \(3\)-factors, and \(\binom{n}{2} – s\) is the number of triply repeated edges in \(3\)-factors. We determine the spectrum of types of \(3\)-factorizations of \(3K_{2n}\), for all \(n \geq 6\); for each \(n \geq 6\), there are \(43\) pairs \((t,s)\) meeting numerical conditions which are not types and all others are types. These \(3\)-factorizations lead to threefold triple systems of different types.