We study the weight distributions of the ternary codes of finite projective planes of order \(9\). The focus of this paper is on codewords of small Hamming weight. We show that there are many weights for which there are no codewords.

For a given sequence of nonincreasing numbers, \(\mathbf{d} = (d_1, \ldots, d_n)\), a necessary and sufficient condition is presented to characterize \(d\) when its realization is a unique labelled simple graph. If \(G\) is a graph, we consider the subgraph \(G’\) of \(G\) with maximum edges which is uniquely determined with respect to its degree sequence. We call the set of \(E(G) \setminus E(G’)\) the smallest edge defining set of \(G\). This definition coincides with the similar one in design theory.

A graph \(G\) without isolated vertices is said to be set-magic if its edges can be assigned distinct subsets of a set \(X\) such that for every vertex \(v\) of \(G\), the union of the subsets assigned to the edges incident with \(v\) is \(X\); such a set-assignment is called a set-magic labeling of \(G\). In this note, we study infinite set-magic graphs and characterize infinite graphs \(G\) having set-magic labelings \(f\) such that \(|f(e)| = 2\) for all \(e \in E(G)\).

A perfect \(\langle k,r \rangle\)-latin square \(A = (a_{i,j})\) of order \(n\) with \(m\) elements is an \(n \times n\) array in which each element occurs in each row and column, and the element \(a_{i,j}\) occurs either \(k\) times in row \(i\) and \(r\) times in column \(j\), or occurs \(r\) times in row \(i\) and \(k\) times in column \(j\). In 1989, Cai, Kruskal, Liu, and Shen studied the existence of perfect \(\langle k,r \rangle\)-latin squares. Here, a simpler construction of perfect \(\langle k,r \rangle\)-latin squares is given.

De Bruijn sequences had been well investigated in \(70s-80s\). In the past, most of the approaches used to generate de Bruijn sequences were based upon either finite field theory or combinatorial theory. This paper describes a simple approach for generating de Bruijn sequences as “seeds”, and then based upon the “seeds”, a simple procedure is presented to reproduce a class of de Bruijn sequences. Numerical results of the distribution of reproduced sequences are provided. Additionally, this paper also reports some recent applications of de Bruijn sequences in psychology and engineering.

A graph \(G(V, E)\) is a mod sum graph if there is a labeling of the vertices with distinct positive integers so that an edge is present if and only if the sum of the labels of the vertices incident on the edge, modulo some positive integer, is the label of a vertex of the graph. It is known that wheels are not mod sum graphs. The mod sum number of a graph is the minimum number of isolates that, together with the given graph, form a mod sum graph. The mod sum number is known for just a few classes of graphs. In this paper we show that the mod sum number of the \(n\)-spoked wheel, \(\rho(W_n)\), \(n \geq 5\), is \(n\) when \(n\) is odd and \(2\) when \(n\) is even.

Kahn (see [3]) reported that N. Alon, M. Saks, and P. D. Seymour made the following conjecture. If the edge set of a graph \(G\) is the disjoint union of the edge sets of \(m\) complete bipartite graphs, then \(\chi(G) \leq m+1\). The purpose of this paper is to provide a proof of this conjecture for \(m \leq 4\) and \(m \geq n – 3\) where \(G\) has \(n\) vertices.

In a graph \(G = (V, E)\), a set \(S\) of vertices (as well as the subgraph induced by \(S\)) is said to be dominating if every vertex in \(V \setminus S\) has at least one neighbor in \(S\). For a given class \(\mathcal{D}\) of connected graphs, it is an interesting problem to characterize the class \({Dom}(\mathcal{D})\) of graphs \(G\) such that each connected induced subgraph of \(G\) contains a dominating subgraph belonging to \(\mathcal{D}\). Here we determine \({Dom}(\mathcal{D})\) for \(\mathcal{D} = \{P_1, P_2, P_5\}\), \(\mathcal{D} = \{K_t \mid t \geq 1\} \cup \{P_5\}\), and \(\mathcal{D} =\) {connected graphs on at most four vertices} (where \(P_t\) and \(K_t\) denote the path and the complete graph on \(t\) vertices, respectively). The third theorem solves a problem raised by Cozzens and Kelleher [\(Discr. Math.\) 86 (1990), 101-116]. It turns out that, in each case, a concise characterization in terms of forbidden induced subgraphs can be given.

We use the results on \(5\)-GDDs to obtain optimal packings with block size five and index one. In particular, we prove that if \(v \equiv 2, 6, 10 \pmod{20}\), there exists an optimal packing with block size five on \(v\) points with at most \(32\) possible exceptions. Furthermore, if \(v \equiv 14, 18 \pmod{20}\), there exists an optimal packing with block size five on \(v\) points with a finite (large) number of possible exceptions.

A chromatic root is a root of the chromatic polynomial of some graph \(G\). E. Farrell conjectured in \(1980\) that no chromatic root can lie in the left-half plane, and in \(1991\) Read and Royle showed by direct computation that the chromatic polynomials of some graphs do have a root there. These examples, though, yield only finitely many such chromatic roots. Subsequent results by Shrock and Tsang show the existence of chromatic roots of arbitrarily large negative real part. We show that theta graphs with equal path lengths of size at least \(8\) have chromatic roots with negative real part.