Let \( U(n, f) \) denote the graph with vertex set the set of unlabeled graphs of order \( n \) that have no vertex of degree greater than \( f \). Two vertices \( H \) and \( G \) of \( U(n, f) \) are adjacent if and only if \( H \) and \( G \) differ (up to isomorphism) by exactly one edge. The problem of determining the values of \( n \) and \( f \) for which \( U(n, f) \) contains a Hamilton path is investigated. There are only a few known non-trivial cases for which a Hamilton path exists. Specifically, these are \( U(5, 3) \), \( U(6, 3) \), and \( U(7, 3) \). On the other hand, there are many cases for which it is shown that no Hamilton path exists. The complete solution of this problem is unresolved.

We study nega-cyclic \(\pm 1\) matrices. We obtain preliminary results which are then used to decrease the search space. We find that there are \( 2 \), \( 4 \), \( 9 \), \( 23 \), \( 63 \), and \( 187 \) ip-equivalence classes for lengths \( 3 \), \( 5 \), \( 7 \), \( 9 \), \( 11 \), and \( 13 \) respectively. The matrices we find are used in a variant given here of the Goethals-Seidel array to form Hadamard matrices, the aim being to later check them for suitability for CDMA schemes.

Fishburn, Tanenbaum, and Trenk define the linear discrepancy \(\text{Id}(P)\) of a poset \( P = (V, <_P) \) as the minimum integer \( k \geq 0 \) for which there exists a bijection \( f : V \rightarrow \{1,2,\ldots,|V|\} \) such that \( u <_P v \) implies \( f(u) < f(v) \) and \( u ||_P v \) implies \( |f(u) – f(v)| \leq k \). In their work, they prove that the linear discrepancy of a poset equals the bandwidth of its cocomparability graph. Here we provide partial solutions to some problems formulated in their study about the linear discrepancy and the bandwidth of cocomparability graphs.

To date, investigations on critical sets for a set of mutually orthogonal Latin squares (MOLS) have been carried out only for small orders \( \leq 9 \). In this paper, we deal with a pair of cyclic orthogonal Latin squares of order \( n \), \( n \geq 11 \), \( n \) odd. Through the construction of a uniquely completable set, we give an upper bound on the size of the minimal critical set. In particular, for \( n = 15 \), a critical set achieving this bound is obtained.

We improve the lower bound for the \(\alpha\)-size of trees with maximum degree three.

A graph \( G \) is called \((a,d)\)-\(\text{edge antimagic total}\) \((a, d)\)-\(\text{EAT}\) if there exist integers \( a > 0, d \geq 0 \) and a bijection \( \lambda: V \cup E \rightarrow \{1,2,\ldots,|V| + |E|\} \) such that \(W = \{w(xy) : xy \in E\} = \{a,a+d,\ldots,a+(|E|-1)d\},\) where \( w(xy) = \lambda(x) + \lambda(y) + \lambda(xy) \). An \((a, d)\)-EAT labeling \( \lambda \) of graph \( G \) is \text{super} if \( \lambda(V) = \{1,2,\ldots,|V|\} \). In this paper, we describe how to construct a super \((a, d)\)-EAT labeling on some classes of disconnected graphs, namely \( P_n \cup P_{n+1} \), \( nP_2 \cup P_n \), and \( nP_2 \cup P_{n+2} \), for positive integer \( n \).

A graph \( G(V, E) \) is called a sum graph if there is an injective labeling called sum labeling \( L \) from \( V \) to a set of distinct positive integers \( S \) such that \( xy \in E \) if and only if there is a vertex \( w \in V \) such that \( L(w) = L(x) + L(y) \in S \). In such a case, \( w \) is called a working vertex. Every graph can be made into a sum graph by adding some isolated vertices, if necessary. The smallest number of isolated vertices that need to be added to a graph \( H \) to obtain a sum graph is called the sum number of \( H \); it is denoted by \( \sigma(H) \). A sum labeling which realizes \( H \cup \overline{K_\sigma(G)} \) as a sum graph is called an optimal sum labeling of \( H \).

Sum graph labeling offers a new method for defining graphs and for storing them digitally. Traditionally, a graph is defined as a set of vertices and a set of edges, specified by pairs of vertices which are the endpoints of an edge. To record a graph on a computer, the edges are usually stored either in the form of an adjacency matrix or as a linked list. Using sum graph labeling, we only need to store the set of vertices, together with some additional isolates, if needed. While previously the edges in a graph were specified explicitly, using sum graphs, edges can be specified implicitly.

A sum labeling \( L \) is called an exclusive sum labeling with respect to a subgraph \( H \) of \( G \) if \( L \) is a sum labeling of \( G \) where \( H \) contains no working vertex. The exclusive sum number \( \epsilon(H) \) of a graph \( H \) is the smallest number \( r \) such that there exists an exclusive sum labeling \( L \) which realizes \( H \cup \overline{K_{r}} \) as a sum graph. A labeling \( L \) is an optimal exclusive sum labeling of a graph \( H \) if \( L \) is a sum labeling of \( H \cup K_{\epsilon(H)} \) and \( H \) contains no working vertex. While the exclusive sum number is never smaller than the corresponding sum number of a graph, labeling graphs exclusively has other desirable features which give greater scope for combining two or more labeled graphs.

In this paper, we introduce exclusive sum graph labeling and we construct optimal exclusive sum graph labeling for complete bipartite graphs, paths, and cycles. The paper concludes with a summary of known results in exclusive sum labeling and exclusive sum numbers for several classes of graphs.

A total vertex irregular labeling of a graph \( G \) with \( v \) vertices and \( e \) edges is an assignment of integer labels to both vertices and edges so that the weights calculated at vertices are distinct. The total vertex irregularity strength of \( G \), denoted by \( tvs(G) \), is the minimum value of the largest label over all such irregular assignments. In this paper, we consider the total vertex irregular labeling of complete bipartite graphs \( K_{m,n} \) and prove that

\[
tvs(K_{m,n}) \geq \max \left\{ \left\lceil \frac{m+n}{m+1} \right\rceil, \left\lceil \frac{2m+n-1}{n} \right\rceil \right\} \quad \text{if } (m,n) \neq (2,2).
\]

For given graphs \( G \) and \( H \), the Ramsey number \( R(G, H) \) is the smallest natural number \( n \) such that for every graph \( F \) of order \( n \): either \( F \) contains \( G \) or the complement of \( F \) contains \( H \).

This paper investigates the Ramsey number \( R(S_n, W_m) \) of stars versus wheels. We show that if \( m \) is odd, \( n \geq 3 \), and \( m \leq 2n-1 \), then \( R(S_n, W_m) = 3n-2 \). Furthermore, if \( n \) is odd and \( n \geq 5 \), then \( R(S_n, W_m) = 3n – \mu \), where \( \mu = 4 \) if \( m = 2n – 4 \) and \( \mu = 6 \) if \( m = 2n – 8 \) or \( m = 2n – 6 \).

The notions of sum labeling and sum graph were introduced by Harary in 1990. In a sum labeling, a vertex is called a working vertex if its label is equal to the sum of the labels of a pair of two distinct vertices.

A sum labeling of a graph \( G \) is said to be \(\text{exclusive}\) if it is a sum labeling of \( G \) such that \( G \) contains no working vertex. Any connected graph \( G \) will require some additional isolated vertices in order to be labeled exclusively. The smallest number of such isolates is called the exclusive sum number of \( G \); it is denoted by \( \epsilon(G) \). The number of isolates cannot be less than the maximum number of neighbors of any vertex in the graph, that is, at least equal to \( \Delta(G) \), the maximum vertex degree in \( G \). If \( \epsilon(G) = \Delta(G) \), then \( G \) is said to be a \( \Delta \)-\(\text{optimum summable graph}\). An exclusive sum labeling of \( G \) using \( \Delta(G) \) isolates is called a \( \Delta \)-optimum exclusive sum labeling of \( G \).

In this paper, we show that some families of trees are \( \Delta \)-optimum summable graphs. However, this is not true for all trees, and we present an example of a tree that is not a \( \Delta \)-optimum summable graph, giving rise to an open problem.