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.

Let \(\text{ASG}(2v,\mathbb{F}_q)\) be the \(2v\)-dimensional affine-symplectic space over the finite field \(\mathbb{F}_q\), and let \(\text{ASp}_{2v}(\mathbb{F}_q)\) be the affine-symplectic group of degree \(2v\) over \(\mathbb{F}_q\). For any two orbits \(M’\) and \(M”\) of flats under \(\text{ASp}_{2v}(\mathbb{F}_q)\), let \(\mathcal{L}’\) (resp. \(\mathcal{L}”\)) be the set of all flats which are joins (resp. intersections) of flats in \(M’\) (resp. \(M”\)) such that \(M” \subseteq L’\) (resp. \(M’ \subseteq \mathcal{L}”\)) and assume the join (resp. intersection) of the empty set of flats in \(\text{ASG}(2v,\mathbb{F}_q)\) is \(\emptyset\) (resp. \(\mathbb{F}_q^{(2v)}\)). Let \(\mathcal{L} =\mathcal{L}’ \cap \mathcal{L}”\). By ordering \(\mathcal{L}’,\mathcal{L}”, \mathcal{L}\) by ordinary or reverse inclusion, six lattices are obtained. This article discusses the relations between different lattices, and computes their characteristic polynomial.

In this paper, we calculate the number of fuzzy subgroups of a special class of non-abelian groups of order \(p^3\).

This paper addresses the problem of capturing nondominated points on non-convex Pareto frontiers, which are encountered in \(E\)-convex multi-objective optimization problems. We define a nondecreasing map \(T\) which transfers a non-convex Pareto frontier to a convex Pareto frontier. An algorithm to find a piecewise linear approximation of the nondominated set of the convex Pareto frontier is applied. Finally, the inverse map of \(T\) is used to obtain the non-convex Pareto frontier.

The aim of our paper is to introduce generalized neighborhood bases and \(gn-T_2\)-spaces. \((\psi, \psi’)\)-continuity, sequentially \((\psi, \psi’)\)-continuity, and \(\psi\)-convergency are investigated on strong generalized first countable spaces, and also two results about \(\psi\)-convergency on \((\psi, \psi’)\)-\(T_2\)-spaces are given.

For a graph \(H\) and an integer \(k \geq 2\), let \(\sigma_k(H)\) denote the minimum degree sum of \(k\) independent vertices of \(H\). We prove that if a connected claw-free graph \(G\) satisfies \(\sigma_{k+1}(G) \geq |G| – k\), then \(G\) has a spanning tree with at most \(k\) leaves. We also show that the bound \(|G| – k\) is sharp and discuss the maximum degree of the required spanning trees.

Define the conditional recurrence sequence \(q_n = aq_{n-1} + bq_{n-2}\) if \(n\) is even, \(q_n = bq_{n-1} + cq_{n-2}\) if \(n\) is odd, where \(q_0 = 0, q_1 = 1\). Then \(q_n\) satisfies a fourth-order recurrence while both \(q_{2n}\) and \(q_{2n+1}\) satisfy a second-order recurrence.

Analogously to a Lucas pseudoprime, we define a composite number \(n\) to be a conditional Lucas pseudoprime (clpsp) if \(n\) divides \(q_{n – (\frac{\Delta}{n})}\), where \(\Delta = a^2 + b^2 + 4ab\) and \((\frac{\Delta}{n})\) denotes the Jacobi symbol. We prove that if \((n, 2ab\Delta) = 1\), then there are infinitely many conditional Lucas pseudoprimes. We also address the question, given an odd composite integer \(n\), for how many pairs \((a, b)\) is \(n\) a conditional Lucas pseudoprime?

Let \(G\) be a simple connected graph with \(n\) vertices. Denoted by \(L(G)\) the Laplacian matrix of G. In this paper, we present a sequence of graphs \({G_n}\) with \(\lim\limits_{n\to \infty} \mu_3(G_n) = 1.5550\) by investigating the eigenvalues of the line graphs of \({G_n}\). Moreover, we prove that the limit is the minimal limit point of the third largest Laplacian eigenvalues of graphs.