Given \(S\) a benzenoid system, we find an expression of the second order Randić index, denoted by \(\mathop{^2\chi}(S)\), in terms of inlet features of \(S\). As a consequence, we classify benzenoid systems with equal \(\mathop{^2\chi}\) and then find the minimal and maximal value over the set of catacondensed systems.

Let \(m_1, m_2, \ldots, m_r\) be positive integers with \(m_i \geq 3\) for all \(i\). An \((m_1, m_2, \ldots, m_r)\)-cycle is defined as the edge-disjoint union of \(r\) cycles of lengths \(2m_1, 2m_2, \ldots, 2m_r\). An \((m_1, m_2, \ldots, m_r)\)-cycle system of the complete graph \(K_n\) is a decomposition of \(K_n\) into \((m_1, m_2, \ldots, m_r)\)-cycles.

In this paper, the necessary and sufficient conditions for the existence of an \((m_1, m_2, \ldots, m_r)\)-cycle system of \(K_n\) are given, where \(m_i\) \((1 \leq i \leq r)\) are odd integers with \(3 \leq m_i \leq n\) and \(\sum_{i=1}^r m_i = 2^k\) for \(k \geq 3\). Moreover, the complete graph with a \(1\)-factor removed \(K_n – F\) has a similar result.

We refer to a labeling of a plane graph as a d-antimagic labeling if the vertices, edges and faces of the graph are labeled in such a way that the label of a face and the labels of vertices and edges surrounding that face add up to a weight of the face and the weights
of faces constitute an arithmetical progression of difference \(d\). In this paper we deal with \(d\)-antimagic labeling of prisms.

We show that the classical Ramsey number \(R(3,3,3,3)\) is no greater than \(62\). That is, any edge coloring with four colors of a complete graph on \(62\) vertices must contain a monochromatic triangle. Basic notions and a historical overview are given along with the theoretical framework underlying the main result. The algorithms for the computational verification of the result are presented along with a brief discussion of the software tools that were utilized.

We show that certain subsets of \(\mathbf{F}_q\)-rational points of the curve \(XZ^{n-1} = Y^n\) are dense sets in \(\mathbf{P}^2(\mathbf{F}_q)\).

H. Kharaghani, in “Arrays for orthogonal designs” \((2000)\), \(\textit{J. Combin. Designs}\), \(8 (2000), 166-173\), showed how to use amicable sets of matrices to construct orthogonal designs in orders divisible by eight. We show how amicable orthogonal designs can be used to make amicable sets and so obtain infinite families of orthogonal designs in six variables in orders divisible by eight.

Let \(G\) and \(H\) be a pair of non-isomorphic graphs on fewer than \(m\) vertices. In this paper, we introduce several new problems about decomposing the complete graph \(K_m\) into copies of \(G\) and \(H\). We will assume that at least one of \(G\) or \(H\) is not a cycle. We also begin to examine variations to the problems of subgraph packing, covering, and factorization.

For vertices \(u\) and \(v\) in a connected graph \(G\) with vertex set \(V\), the distance \(d(u,v)\) is the length of a shortest \(u – v\) path in \(G\). A \(u – v\) path of length \(d(u,v)\) is called a \(u – v\) geodesic. The closed interval \(I[u,v]\) consists of \(u\), \(v\), and all vertices that lie in some \(u – v\) geodesic of \(G\); while for \(S \subseteq V\), \(I[S]\) is the union of closed intervals \(I[u,v]\) for all \(u,v \in S\). A set \(S\) of vertices is a geodetic set if \(I[S] = V\), and the minimum cardinality of a geodetic set is the geodetic number \(g(G)\). For vertices \(x\) and \(y\) in \(G\), the detour distance \(D(x, y)\) is the length of a longest \(x – y\) path in \(G\). An \(x – y\) path of length \(D(x, y)\) is called an \(x – y\) detour. The closed detour interval \(I_D[x,y]\) consists of \(x\), \(y\), and all vertices in some \(x – y\) detour of \(G\). For \(S \subseteq V\), \(I_D[S]\) is the union of \(I_D[x,y]\) for all \(x,y \in S\). A set \(S\) of vertices is a detour set if \(I_D[S] = V\), and the minimum cardinality of a detour set is the detour number \(dn(G)\). We study relationships that can exist between minimum detour sets and minimum geodetic sets in a graph. A graph \(F\) is a minimum detour subgraph if there exists a graph \(G\) containing \(F\) as an induced subgraph such that \(V(F)\) is a minimum detour set in \(G\). It is shown that \(K_3\) and \(P_3\) are minimum detour subgraphs. It is also shown that for every pair \(a,b \geq 2\) of integers, there exists a connected graph \(G\) with \(dn(G) = a\) and \(g(G) = b\).