A spectrally arbitrary pattern \({A}\) is a sign pattern of order \(n\) such that every monic real polynomial of degree \(n\) can be achieved as the characteristic polynomial of a matrix with sign pattern \({A}\). A sign pattern \({A}\) is minimally spectrally arbitrary if it is spectrally arbitrary but is not spectrally arbitrary if any nonzero entry (or entries) of \({A}\) is replaced by zero. In this paper, we introduce some new sign patterns which are minimally spectrally arbitrary for all orders \(n\geq 7\).

Let \(G\) be a graph with vertex-set \(V = V(G)\) and edge-set \(E = E(G)\), and let \(e = |E(G)|\) and \(v = |V(G)|\). A one-to-one map \(\lambda\) from \(V \cup E\) onto the integers \(\{1, 2, \ldots, v+e\}\) is called a vertex-magic total labeling if there is a constant \(k\) so that for every vertex \(x\),

\[\lambda(x) + \sum \lambda(xy) = k\]

where the sum is over all edges \(xy\) where \(y\) is adjacent to \(x\). Let us call the sum of labels at vertex \(x\) the weight \(w_\lambda\) of the vertex under labeling \(\lambda\); we require \(w_\lambda(x) = k\) for all \(x\). The constant \(k\) is called the magic constant for \(\lambda\).

A sun \(S_n\) is a cycle on \(n\) vertices \(C_n\), for \(n \geq 3\), with an edge terminating in a vertex of degree \(1\) attached to each vertex.

In this paper, we present the vertex-magic total labeling of the union of suns, including the union of $m$ non-isomorphic suns for any positive integer $m \geq 3$, proving the conjecture given in [6].

The Randić index of an organic molecule whose molecular graph is \(G\) is the sum of the weights \((d(u)d(v))^{1/2}\) of all edges \(uv\) of \(G\), where \(d(u)\) denotes the degree of the vertex \(u\) of the molecular graph \(G\). Among all trees with \(n\) vertices and \(k\) pendant vertices, the extremal trees with the minimum, the second minimum, and the third minimum Randić index were characterized by Hansen, Li, and Wu \(et al\)., respectively. In this paper, we further investigate some small Randić index properties and give other elements of small Randić index ordering of trees with \(k\) pendant vertices.

Consider a complete graph of multiplicity \(2\), where between every pair of vertices there is one red and one blue edge. Can the edge set of such a graph be decomposed into isomorphic copies of a \(2\)-coloured path of length \(2k\) that contains \(k\) red and\(k\) blue edges? A necessary condition for this to be true is \(n(n-1) \equiv 0 \mod k\). We show that this is sufficient for \(k \leqq 3\).

In this paper, we investigate super-simple cyclic \((v, k, \lambda)\)-BIBDs (SCBIBs). Some general constructions for SCBIBs are given. The spectrum of super-simple cyclic \((v, 3, \lambda)\) is completely determined for \(\lambda = 2, 3\) and \(v – 2\). From that, some new optical orthogonal codes are obtained.

The cycle structure of a Latin square autotopism \(\Theta = (\alpha, \beta, \gamma)\) is the triple \((I_\alpha,I_\beta, I_\gamma)\), where \(I_\delta\) is the cycle structure of \(\delta\), for all \(\delta \in \{\alpha, \beta, \gamma\}\). In this paper, we study some properties of these cycle structures and, as a consequence, we give a classification of all autotopisms of the Latin squares of order up to \(11\).

This work presents explicit expressions of the \(3\)-restricted edge connectivity of Cartesian product graphs, which yields some sufficient conditions for the product graphs to be maximally \(3\)-restricted edge connected.

Dirac characterized chordal graphs by every minimal \((2\)-)vertex separator inducing a complete subgraph. This generalizes to \(k\)-vertex separators and to a characterization of the class of \(\{P_5, 2P_3\}\)-free chordal graphs. The correspondence between minimal \(2\)-vertex separators of chordal graphs and the edges of their clique trees parallels a correspondence between minimal \(k\)-vertex separators of \(\{P_5, 2P_3\}\)-free chordal graphs and certain \((k-1)\)-edge substars of their clique trees.

It is well known that the Petersen graph does not contain a Hamilton cycle. In \(1983\), Alspach completely determined which Generalized Petersen graphs are Hamiltonian \([1]\). In this paper, we define a larger class of graphs which includes the Generalized Petersen graphs as a special case, and determine which graphs in this larger class are Hamiltonian, and which are \(1\)-factorable. We call this larger class spoked Cayley graphs.

Let \(K_v\) be the complete graph with \(v\) vertices, where any two distinct vertices \(x\) and \(y\) are joined by exactly one edge \(\{x,y\}\). Let \(G\) be a finite simple graph. A \(G\)-design of \(K_v\), denoted by \((v,G,1)\)-GD, is a pair \((X,\mathcal{B})\), where \(X\) is the vertex set of \(K_v\), and \(\mathcal{B}\) is a collection of subgraphs of \(K_v\), called blocks, such that each block is isomorphic to \(G\) and any two distinct vertices in \(K_v\) are joined in exactly one block of \(\mathcal{B}\). In this paper, the discussed graphs are \(G_i\), \(i = 1,2,3,4\), where \(G_i\) are the four graphs with 7 points, 7 edges, and a 5-cycle. We obtain the existence spectrum of \((v, G_i,1)\)-GD.