Let \(G = (V, E)\) be a simple graph. A paired-dominating set of a graph \(G\) is a dominating set whose induced subgraph contains a perfect matching. The paired domination number of a graph \(G\), denoted by \(\gamma_p(G)\), is the minimum cardinality of a paired-dominating set in \(G\). In this paper, we study the paired domination number of generalized Petersen graphs \(P(n,2)\) and prove that for any integer \(n \geq 6\), \(\gamma_p(P(n, 2)) = 2 \left\lfloor \frac{n}{3} \right\rfloor + n \pmod{3}\).

The Estrada index of a simple connected graph \(G\) of order \(n\) is defined as \(EE(G) = \sum_{i=1}^{n} e^{\lambda_i}\), where \(\lambda_1, \lambda_2, \ldots, \lambda_n\) are the eigenvalues of the adjacency matrix of \(G\). In this paper, we characterize all pentacyclic graphs of order \(n\) with maximal Estrada index.

Let \(\Pi\) be a finite polar space of rank \(n \geq 2\) fully embedded into a projective space \(\Sigma\). In this note, we determine all tight sets of \(\Pi\) of the form \((\Sigma_1 \cap \mathcal{P}) \cup (\Sigma_2 \cap \mathcal{P})\), where \(\mathcal{P}\) denotes the point set of \(\Pi\) and \(\Sigma_1, \Sigma_2\) are two mutually disjoint subspaces of \(\Sigma\). In this way, we find two families of \(2\)-tight sets of elliptic polar spaces that were not described before in the literature.

In this paper, we define a new matrix identity for bi-periodic Fibonacci and Lucas numbers. By using the matrix method, we give simple proofs of several properties of these numbers. Moreover, we obtain a new binomial sum formula for bi-periodic Fibonacci and Lucas numbers, which generalize the former results.

Hein and Sarvate show how to decompose \(\lambda\) copies of a complete graph \(K_n\), for some minimal value of \(\lambda\), into so-called LOE and OLE graphs. In this paper, we will show that for all possible values of \(\lambda\), the necessary conditions are sufficient for the LOE and OLE decompositions.

Let \(R\) be a commutative ring. The regular digraph of ideals of \(R\), denoted by \(\mathcal{R}(R)\), is a digraph whose vertex-set is the set of all non-trivial ideals of \(R\) and, for every two distinct vertices \(I\) and \(J\), there is an arc from \(I\) to \(J\), whenever \(I\) contains a non-zero divisor of \(J\). In this paper, we investigate the planarity of \(\mathcal{R}(R)\). We also completely characterize the rings \(R\) such that \(\mathcal{R}(R)\) is a ring graph, and the situations under which the genus of \(\mathcal{R}(R)\) is finite. Moreover, we study the independence number and the girth of \(\mathcal{R}(R)\), and also we find all cases that \(\mathcal{R}(R)\) is bipartite.

In this paper, the existence of Yang Hui type magic squares of order \(n\) with \(t\)-powered sum (YMS(\(n\), \(t\))) for general \(t\) is investigated. Some constructions of YMS(\(n\), \(t\)) are obtained by using strongly symmetric self-orthogonal diagonal Latin squares and magic rectangles. Applying these constructions, it is proved that for an integer \(t > 1\) there exist both a symmetric elementary YMS(\(2^t\), \(2t – 2\)) and a symmetric elementary YMS(\(2^t – k\), \(2t\)) for odd \(k > 1\), which improves the known result on YMSs.

To gain a better understanding of clean rings and their relatives, the clean graph of a commutative ring with identity is introduced and its various properties are established. Further investigation of clean graphs leads to additional results concerning other classes of rings.

For a connected graph, the distance spectral radius is the largest eigenvalue of its distance matrix. In this paper, of all trees with both given order and fixed diameter, the trees with the minimal distance spectral radius are completely characterized.

In this paper, a domination-type parameter, called dynamical \(2\)-domination number, will be introduced. Let \(G = (V(G), E(G))\) be a graph. A subset \(D \subseteq V(G)\) is called a \(2\)-dominating set in \(G\) if every vertex in \(V(G) \setminus D\) is adjacent to at least two vertices in \(D\), and in this paper \(D\) is called a dynamical \(2\)-dominating set if there exists a sequence of sets \(D = V_0 \subseteq V_1 \subseteq V_2 \subseteq \cdots \subseteq V_k = V(G)\) such that, for each \(i\), \(V_{i-1}\) is a \(2\)-dominating set in \(\langle V_i \rangle\), the induced subgraph generated by \(V_i\). Also, for a given graph \(G\), the size of its dynamical \(2\)-dominating sets of minimum cardinality will be called the dynamical \(2\)-domination number of \(G\) and will be denoted by \(\bar{\gamma}_{2}(G)\). We study some basic properties of dynamical \(2\)-dominating sets and compute \(\bar{\gamma}_{2}(G)\) for some graph classes. Also, some results about \(\bar{\gamma}_{2}\) of a number of binary operations on graphs are proved. A characterization of graphs with extreme values of \(\bar{\gamma}_{2}\) is presented. Finally, we study this concept for trees and give an upper bound and a lower bound for the dynamical \(2\)-domination number of trees.