The Wiener index, one of the oldest molecular topological descriptors used in mathematical chemistry, was well-studied during the past decades. For a graph \(G\), its Wiener index is defined as \(W(G) = \sum\limits_{\{u, v\} \subseteq V(G)} d_G(u, v)\), where \(d_G(u, v)\) is the distance between two vertices \(u\) and \(v\) in \(G\). In this paper, we study the Wiener index of a class of composite graph, namely, double graph. We reveal the relation between the Wiener index of a given graph and the one of its double graph as well as the relation between Wiener index of a given graph and the one of its \(k\)-iterated double graph. As a consequence, we determine the graphs with the maximum and minimum Wiener index among all double graphs and \(k\)-iterated double graphs of connected graphs of the same order, respectively.

The set of unicyclic graphs with \(n\) vertices and diameter \(d\) is denoted by \(\mathcal{U}_{n,d}\). For \(3 \leq i \leq d\), let \(P_{n-d-1}(i)\) be the graph obtained from path \(P_{d+1}: v_1 v_2 \ldots v_{d+1}\) by adding \(n-d-1\) pendant edges at \(v_i\), and \(U_{n-d-2}(i)\) be the graph obtained from \(P_{n-d-1}(i)\) by joining \(v_{i-2}\) and a pendant neighbor of \(v_{i}\). In this paper, we determine all unicyclic graphs in \(\mathcal{U}_{n,d}\) whose largest Laplacian eigenvalue is greater than \(n-d+2\). For \(n-d \geq 6\) and \(G \in \mathcal{U}_{n,d}\), we prove further that the largest Laplacian eigenvalue \(\mu(G) \leq \max\{\lambda(U_{n,d-2}(i)) \mid 3 \leq i \leq d\}\), and conjecture that \(\mathcal{U}_{n,d}.\) is the unique graph which has the greatest value of the greatest Laplacian eigenvalue in \(\mathcal{U}_{n,d}\). We also prove that the conjecture is true for \(3 \leq d \leq 6\).

The Padmakar-Ivan \((PI)\) index is a Wiener-Szeged-like topological index which reflects certain structural features of organic molecules. In this paper, we study the PI index with respect to the extremal simple pericondensed hexagonal systems and we solve it completely.

Let \(\lambda K_v\) be the complete multigraph with \(v\) vertices. Let \(G\) be a finite simple graph. A \(G\)-design (\(G-GD_\lambda)(v)\) (\(G\)-packing (\(G-PD_\lambda)(v)\), \(G\)-covering (\(G-CD_\lambda)(v)\)) of \(K_v\) 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 exactly (at most, at least) in \(\lambda\) blocks. In this paper, we will discuss the maximum packing designs and the minimum covering designs for four particular graphs each with six vertices and nine edges.

Let \(a\) and \(b\) be integers such that \(1 \leq a < b\), and let \(G\) be a graph of order \(n\) with \(n \geq \frac{(a+b)(2a+2b-3)}{a+1}\) and the minimum degree \(\delta(G) \geq \frac{(b-1)^2-(a+1)(b-a-2)}{a+1} \). Let \(g(x)\) and \(f(x)\) be two nonnegative integer-valued functions defined on \(V(G)\) such that \(a \leq g(x) \leq f(x) \leq b\) for each \(x \in V(G)\). We prove that if \(|N_G(x) \cup N_G(y)| \geq \frac{(b-1)n}{a+b} \) for any two nonadjacent vertices \(x\) and \(y\) in \(G\), then \(G\) has a \((g, f)\)-factor. Furthermore, it is shown that the result in this paper is best possible in some sense.