Molecular Topology of GraphsView this Special Issue
On the Reformulated Second Zagreb Index of Graph Operations
Topological indices (TIs) are expressed by constant real numbers that reveal the structure of the graphs in QSAR/QSPR investigation. The reformulated second Zagreb index (RSZI) is such a novel TI having good correlations with various physical attributes, chemical reactivities, or biological activities/properties. The RSZI is defined as the sum of products of edge degrees of the adjacent edges, where the edge degree of an edge is taken to be the sum of vertex degrees of two end vertices of that edge with minus 2. In this study, the behaviour of RSZI under graph operations containing Cartesian product, join, composition, and corona product of two graphs has been established. We have also applied these results to compute RSZI for some important classes of molecular graphs and nanostructures.
In the whole study, we only consider the molecular graph [1, 2], a graphical representation of molecular structure, in which every vertex corresponds to the atoms and the edges to the bonds between them. Assume J as a simple (molecular) graph with vertex set and edge set. The notations and represent the number of elements of J in and , respectively. Also, denotes the degree of a vertex (x) in J and is defined as the number of edges incident to x.
TIs can be expressed by real numbers related to graphs. There exist many applications as tools for modelling chemical and other properties of molecules for TIs. They determine the correlation between the specific properties of molecules and the biological activity with their configuration in the study of quantitative structure-activity relationships (QSARs) and quantitative structure-property relationships (QSPRs) . To develop the scientific knowledge in 20th century, the concept of molecular structure plays an important role in chemical graph theory, a branch of mathematical chemistry which is closely related to chemical graph. The molecular structure descriptor, namely, topological index expresses the numerical value obtained from the molecular graph that represents its topology and is necessarily invariant under the automorphism of graphs.
The Zagreb indices, namely, first Zagreb index  and second Zagreb index  were introduced by Gutman et al. in 1972 and 1975, respectively. These two indices are, respectively, defined for molecular graph (J) as
Let x, y, and z be its three vertices of J forming a path of length two. If x and y are two adjacent vertices of J, we express as and similarly for y and z as . The edge connecting these vertices will be denoted by . Then, the three auxiliary Zagreb-type indices are introduced by Basavanagoud et al.  in 2015. They are denoted as follows:
In 2004, Milicevic et al.  reformulated the Zagreb indices by replacing vertex degree with edge degree, and the edge degree of an edge is defined as . The first and second reformulated Zagreb indices  of a graph J are defined aswhere means that the edges e and f share a common end vertex is, and e and f are adjacent. In 2015, Furtula and Gutman  introduced forgotten index (F-index) and is defined as
In mathematical chemistry, graph operations perform a significant role in the formation of new classes of graphs. By different graph operations on some general or particular graphs, some chemically interesting graphs can be obtained. In , Khalifeh et al. computed the first and second Zagreb indices under some graph operations. Some explicit formulae of Zagreb coindices under some graph operations were presented by Ashrafi et al. . In , Das et al. derived multiplicative Zagreb indices of different graph operations. In , De et al. computed the reformulated first Zagreb index under some graph operations. Recently, the analytical expressions for various topological indices under some binary graph operations have been discussed in [14–16]. We also refer to [17–23] in this regard for interested readers.
2. Main Results
In the following, we study different binary graph operations such as join, Cartesian product, composition, and corona product of two molecular graphs and compute some exact formulae for RSZI with respect to those operations separately. Suppose and be two molecular graphs with the vertex sets , , such that , , and the edge sets , , such that , , respectively. We consider the notations , , and for path, cycle, and complete graph with vertices. To establish the main results, we also follow equations (1)–(8).
The join  of two graphs and , denoted by , contains the vertex set and the edge set .
In the following theorem, we compute RSZI for join of two graphs.
Theorem 1. Let be the join of and graphs. Then, RSZI of J is given by
Proof. Consider . Let and be two graphs with , vertices and , edges, respectively. Then, by Table 1, we obtainFirst part:Second part:Third part:Fourth part:Fifth part:Sixth part:By adding , we get the desired result.
The suspension of a graph H is the join or sum of H with a single vertex .
Corollary 1. The RSZI of suspension of H that contain and can be expressed as .
Example 1. The cone graph is defined as . Then, .
Example 2. The RSZI of suspension of graph such as are expressed in Table 2.
Example 3. The complete bipartite graph is the join of . Using Theorem 1, .
2.3. The Cartesian Product
The Cartesian product (CP)  of and , denoted by , is a graph with and any if and only if or . Also, and .
Now, we obtain RSZI for Cartesian product of two graphs.
Theorem 2. If be the CP of and graphs, then RSZI of J is
Proof. By definition of RSZI, from equation (7) and the degree distribution for CP of two graphs, we haveThe notations , and represent the sum of above terms in order. Step 1: Step 2: Step 3: By adding , we get the required result.
Let P, Q, R, and S be the grids , rook’s graph , -nanotorus , and -nanotube . Then, by Theorem 2, we get the following results.
Example 4. The RSZI for P is given by , for .
Example 5. The RSZI for Q is given by , for .
Example 6. The RSZI for R is given by .
Example 7. The RSZI for S is given by .
2.5. Lexicographic Product
The lexicographic product (LP) or composition  of two graphs and is denoted by , and any two vertices and are adjacent if and only if or and . The vertex set of is , and the degree of a vertex is given by .
In the following theorem, we compute RSZI for composition of two graphs and .
Theorem 3. Let be the composition of and . The RSZI of J is given by
Proof. By using the definition of RSZI and from the equation (7), we haveNow,