It is shown how any integral monoid can be represented as the projection of the intersection of the solution set of a finite collection of linear inequalities, and a lattice, both in a possibly higher dimension. This in turn can be used to derive a known representation using Chvátal functions, in the same dimension as the monoid. Both representations can be regarded as discrete analogues of the classical theorems of Weyl and Minkowski, but applicable in non-polyhedral monoids.

Both the bandwidth and additive bandwidth of a graph supply information about the storage requirements of a representation of the graph. In particular, the bandwidth measures how far \(1\)’s must be from the main diagonal of the graph’s adjacency matrix, while the additive bandwidth yields the same information with respect to the main contradiagonal. Thus, storage can be significantly reduced from that required by the full adjacency matrix if at least one of the two types of bandwidths is small, which is most likely to occur for sparse matrices. Alternatively, one could store a representation of the complement of the graph if one of its two bandwidths is small. We relate the additive bandwidth to other graphical invariants and then concentrate on Nordhaus-Gaddum type results to show that there are graphs for which both the bandwidth and the additive bandwidth of both the graph and its complement are large. In other words, some graphs require near maximum storage.

A star-factor of a graph \(G\) is a spanning subgraph of \(G\) such that each component of which is a star. In this paper, we study the structure of the graphs with a unique star-factor and obtain an upper bound on the mnumber of edges such graphs can have. We also investigate the number of star-factors in a regular graph.

Let \(J[v]\) denote the set of numbers \(k\) such that there exist two semi-symmetric Latin squares (SSLS) of order \(v\) which have \(k\) entries in common. In this paper, we show that \begin{align*}
J[3] &= \{0, 9\}, J[4] = \{0, 1, 3, 4, 9, 12, 16\}, \\
J[5] &= \{0, 1, 3, 4, 6, 7, 9, 10, 12, 13, 15, 18, 21, 25\}, \\
J[6] &= \{0, 1, 2, \ldots, 23, 24, 26, 27, 28, 29, 32, 36\}, \text{ and} \\
J[v] &= \{0, 1, 2, \ldots, v^2\} \setminus \{v^2-1, v^2-2, v^2-3, v^2-5, v^2-6\}
\end{align*}
for each \(v \geq 7\).

A holey perfect Mendelsohn design of type \(h_1^{n_1} h_2^{n_2} \ldots h_k^{n_k}\) (HPMD\((h_1^{n_1} h_2^{n_2} \ldots h_k^{n_k})\)), with block size four is equivalent to a frame idempotent quasigroup satisfying Stein’s third law with the same type, where a frame quasigroup of type \(h_1^{n_1} h_2^{n_2} \ldots h_k^{n_k}\) means a quasigroup of order \(n\) with \(n_i\) missing subquasigroups (holes) of order \(h_i\), \(1 \leq i \leq k\), which are disjoint and spanning, that is \(\sum_{1\leq i \leq k} n_ih_i = n\). In this paper, we investigate the existence of HPMD\((2^n3^1)\) and show that an HPMD\((2^n3^1)\) exists if and only if \(n \geq 4\). As an application, we readily obtain HSOLS\((2^n3^1)\) and establish the existence of \((2,3,1)\) [or \((3,1,2)\)]-HCOLS\((2^n3^1)\) for all \(n \geq 4\).

Much of chordal graph theory and its applications is based on chordal graphs being the intersection graphs of subtrees of trees. This suggests also looking at intersection graphs of subgraphs of chordal graphs, and so on, with appropriate conditions imposed on the subgraphs.This paper investigates such a hierarchy of generalizations of “chordal-type” graphs, emphasizing the so-called “ekachordal graphs” — those next in line beyond chordal graphs. Parts of the theory of chordal graphs do carry over to chordal-type graphs, including a recursive, elimination characterization for ekachordal graphs.

We present a new construction to obtain frames with block size four using certain skew Room frames. The existence results of Rees and Stinson for frames with block size four are improved, especially nfor hole sizes divisible by \(6\). As a by-product of the skew Room frames we construct, we are also able to show that a resolvable \((K_4 – e)\)-design with \(60t + 16\) points exists if \(t \geq 0\) and \(t \neq 8, 12\).

It has been proved that the smallest rectangular board on which a \( (p, q) \)-knight’s graph is connected has sides \( p+q \) by \( 2q \) when \( p < q \). It has also been proved that these minimal connected knight's graphs are of genus \( 0 \) or \( 1 \), and that they are of genus \( 0 \) when \( p \) and \( q \) are of the form \( Md+1 \) and \( (M+1)d+1 \), with \( M \) a non-negative integer and \( d \) a positive odd integer. It is proved in this paper that the minimal connected knight's graph is of genus \( 1 \) in all other cases.

In this paper, we study the minimum co-operative guards problem, a variation of the art gallery problem. First, we show that the minimum number of co-operative guards required for a \(k\)-spiral polygon is at most \(N_k\), the total number of reflex vertices in the \(k\)-spiral. Then, we classify \(2\)-spirals into seven different types based on their structure. Finally, we present a minimum co-operative guard placement algorithm for general
\(2\)-spirals.

In this paper, we construct all symmetric \(27, 13, 6\) designs with a fixed-point-free automorphism of order \(3\). There are \(22\) such designs.