#### Abstract

Graph operations play an important role in constructing complex network structures from simple graphs. Computation of topological indices of these complex structures via graph products is an important task. In this paper, we generalized the concept of subdivision double-corona product of graphs and investigated the exact expressions of the first and second Zagreb indices, first reformulated Zagreb index, and forgotten topological index (F-index) of this graph operation.

#### 1. Introduction

In mathematics [1, 2], graph theory is the study of graphs which are mathematical structures used to model pairwise connection between objects. Graph theory presents efficient and powerful tools for the topological characterization of the underline structure. Any problem which includes graph structure can be solved by using the graph theoretical approach. This enables researchers to apply graph theory in various fields such as software engineering, biology, chemistry, and operation research.

We start by defining some basic notions related to graph theory. Let be a simple connected graph with vertex set and edge set . The number of vertices and number of edges of are called its order and size, respectively. A graph of order and size will be denoted by . For any vertex , the degree of vertex is the number of edges incident on the vertex , and it is written as or simply . The subdivision of the graph is denoted by and is obtained by inserting a new vertex on every edge of . For more details on the basic terminologies related to graph theory, we suggest readers to see the book by Gross et al. .

Topological indices are numerical numbers assigned to a molecular graph. These numbers are invariant under graph isomorphism. Topological index is also called a molecular structure descriptor or graph theoretical descriptor . One of the important uses of topological indices is in the development of quantitative structure-activity relationships (QSARs) in which the biological activity or other properties of molecules are correlated with their chemical structure. Topological indices are divided into three categories: degree based, distance based, and counting related. Among these, the most studied ones are degree-based topological indices. The first and second Zagreb indices of a graph are defined as

In 1972, these topological indices were applied for the first time to find the total electron energy of molecular graphs . Later, the Zagreb indices developed important applications in QSPR/QSAR studies, and a lot of research papers have been published on these .

Miličević et al.  reformulated the Zagreb indices in terms of edge degree which are defined aswhere denotes the degree of an edge and is defined as . The notation shows that the edges and are adjacent to each other.

The forgotten topological index or F-index is defined as [13, 14]

In 2013, Shirdel et al.  proposed a new version of Zagreb indices called the first hyper Zagreb index and is defined as

A large number of the topological indices have been discovered and intend to demonstrate substance, drug, and different properties of atoms.

Let , and be three graphs. The subdivision double-corona product of , and is denoted by and is obtained by taking one copy of , copies of , and copies of and joining the i-th old vertex of to every vertex of of the -th copy of and -th new vertex of to every vertex of of the -th copy of . In the following, we generalize the subdivision double-corona product. Let and be two simple connected graphs. It is supposed that we have two further simple connected graphs and . Let and ; then, the generalized subdivision double corona is denoted by and is obtained by taking one copy of , copies of , and copies of and then by joining the -th old vertex of to every vertex of of the -th copy of and -th new vertex of to every vertex of of the -th copy of . Figure 1 illustrates the definition by considering and .

The graph operation, especially graph products, plays a significant role not only in pure and applied mathematics but also in computer science. For instance, the Cartesian product provides a significant model for connecting computers. In order to synchronize the work of the entire framework, it is necessary to search for Hamiltonian paths and cycles in the network . Some graph operations and their topological indices are presented in .

#### 2. Main Results

In this section, we present the main results. Llemma 1 is useful to obtain the main results and can be directly obtained from the definition of the generalized subdivision corona product.

Lemma 1. Let and be two simple connected graphs. Furthermore, it is supposed that we have two simple connected graphs, and . Let and ; then, the degree of vertices in the graph iswhere and .

Theorem 2. Let be a graph and be its subdivision graph. Furthermore, it is supposed that and are two simple graphs. We assume and ; then, the first Zagreb index of is given as

Proof. From the definition of the first Zagreb index, we haveApplying Lemma 1, we getwhich is our required result.

The classical subdivision double-corona product can be obtained as a corollary of Theorem 2 by setting and .

Corollary 1. Let be a graph and be its subdivision graph. Furthermore, it is supposed that and are two simple graphs. We assume and ; then,

Example 1. Let denote a cycle on vertices and denote a path on vertices. Then, by using the statement of Theorem 2, we get

Theorem 3. Let be a graph and be its subdivision graph. Furthermore, it is supposed that and are two simple graphs. We assume and ; then, the second Zagreb index of is given as

Proof. From the definition of the second Zagreb index, we haveNow, applying Lemma 1, we getwhich is our required result.

The classical subdivision double-corona product can be obtained as a corollary by setting and .

Corollary 2. Let be a graph and be its subdivision graph. Furthermore, it is supposed that and are two simple graphs. We assume and ; then,

Example 2. Let denote a cycle on vertices and denote a path on vertices. Then, by using the statement of Theorem 3, we get

Theorem 4. Let be a graph and be its subdivision graph. Furthermore, it is supposed that and are two simple graphs. We assume and ; then, the first reformulated Zagreb index of is given as

Proof. From the definition of the first reformulated Zagreb index, we haveApplying Lemma 1, we get