Let \(\mathcal{G}(p)\) denote the class of simple graphs of order \(p\). For a graph \(G\), the complement of \(G\) is denoted by \(\overline{G}\). For a positive integer \(n\), the \(n\)-path-chromatic number \(\chi_n(G)\) is the least number of colours that can be associated to the vertices of \(G\) such that not all the vertices on any path of length \(n\) receive the same colour. The Nordhaus-Gaddum Problem for the \(n\)-path-chromatic number of \(G\) is to find bounds for \(\chi_n(G) + \chi_n(\overline{G})\) and \(\chi_n(G) \cdot \chi_n(\overline{G})\) over the class \(\mathcal{G}(p)\). In this paper, we determine sharp lower bounds for the sum and the product of \(\chi_n(G)\) and \(\chi_n(\overline{G})\). Furthermore, we provide weak upper bounds for \(\chi_2(G) + \chi_2(\overline{G})\) and \(\chi_2(G) \cdot \chi_2(\overline{G})\).

In this paper, we prove that the Equitable \(\Delta\)-Coloring Conjecture holds for planar graphs with maximum degree \(\Delta \geq 13\).

An array \(A[i, j]\), \(1 \leq i \leq n, 1 \leq j \leq n\), has a period \(A[p,p]\) of dimension \(p \times p\) if \(A[i, j] = A[i+p, j+p]\) for \(i, j = 1 \ldots n-p\). The period of \(A_{p,p}\) is the shortest such \(p\).

We study two-dimensional pattern matching, and several other related problems, all of which depend on finding the period of an array.

In summary, finding the period of an array in parallel using \(p\) processors for general alphabets has the following bounds:

\[
\begin{cases}
\Theta\left(\frac{n^2}{p}\right) & \text{if } p \leq \frac{n^2}{\log \log n}, n>17^3 \quad\quad\quad\quad\quad\quad\quad\quad(1.1) \\
\Theta(\log\log n) & \text{if } \frac{n^2}{\log \log n} < p 17^3 \quad\;\; \quad\quad\quad\quad (1.2) \\
\Theta\left(\log\log_{\frac{2p}{n^2}}{p}\right) & \text{if } n^2 \leq p 17^3 \quad \quad\quad\quad\quad (1.3) \\
\Theta\left(\log\log_{\frac{2p}{n^2}}{p}\right) & \text{if } n^2 \log^2 n \leq p \leq n^4, \text{ $n$ large enough} \quad (1.4)
\end{cases}
\]

In [5] Kløve gave tables of the best bounds known on the size of optimal difference triangle sets. In this note, we give examples of difference triangle sets found by computer search which improve on the upper bounds in [5]. In four cases, these examples are proved to be optimal.

A Latin square \((S, \cdot)\) is said to be \((3, 1, 2)\)-\emph{conjugate-orthogonal} if \(x \cdot y = z \cdot w\), \(x \cdot_{312} y = z \cdot_{312} w\) imply \(x = z\) and \(y = w\), for all \(x, y, z, w \in S\), where \(x_3 \cdot_{312} x_1 = x_2\) if and only if \(x_1 \cdot x_2 = x_3\).

Such a Latin square is said to be \emph{holy} \(((3,1,2)\)-HCOLS for short) if it has disjoint and spanning holes corresponding to missing sub-Latin squares.

Let \((3,1,2)\)-HCOLS\((h^n)\) denote a \((3,1,2)\)-HCOLS of order \(hn\) with \(n\) holes of equal size \(h\).

We show that, for any \(h \geq 1\), a \((3,1,2)\)-HCOLS\((h^n)\) exists if and only if \(n \geq 4\), except \((n,h) = (6,1)\), and except possibly \((n,h) = (10,1)\) and \((4,2t+1)\) for \(t \geq 1\).

Let \((3,1,2)\)-ICOILS\((v,k)\) denote an idempotent \((3,1,2)\)-COLS of order \(v\) with a hole of size \(k\).

We prove that a \((3,1,2)\)-ICOILS\((v,k)\) exists for all \(v \geq 3k+1\) and \(1 \leq k \leq 5\), except possibly \(k = 4\) and \(v \in \{35, 38\}\).

The main object of this paper is the construction of BIBD’s with \(6 \leq k \leq 11\) and \(\lambda = 1\). These balanced incomplete block designs can be simply constructed from some associated group divisible designs with the number of groups being a prime power, and it is these group divisible designs that are constructed directly. Other related designs are discussed.

We have carried out a large number of computer searches for the base sequences \(BS(n + 1, n)\) as well as for three important subsets known as Turyn sequences, normal sequences, and near-normal sequences. In the Appendix, we give an extensive list of \(BS(n + 1, n)\) for \(n \leq 32\). The existence question for Turyn sequences in \(BS(n + 1, n)\) was resolved previously for all \(n \leq 41\), and we extend this bound to \(n \leq 51\). We also show that the sets \(BS(n + 1, n)\) do not contain any normal sequences if \(n = 27\) or \(n = 28\). To each set \(BS(n + 1, n)\), we associate a finite graph \(\Gamma_{n}\), and determine these graphs completely for \(n \leq 27\). We show that \(BS(m,n) = \emptyset \quad \text{if} \quad m \geq 2n, \; n > 1, \; \text{and} \; m + n \; \text{is odd}\),
and we also investigate the borderline case \(m = 2n – 1\).