Received 21 March 2016; accepted 20 May 2016; published 23 May 2016
By a graph, we mean a finite, undirected, simple graph. We denote the vertex set and the edge set of a graph G by V(G) and E(G), respectively. And the number of first neighbors of vertex u in G (the degree of u) is denoted by d(u). For notation and graph theory terminology not presented here, we follow  -  . All of the graphs in this paper are simple and a topological index of a graph is a number related to a graph which is invariant under graph automorphisms and is a numeric quantity from the structural graph of a molecule.
One of the best known and widely used is the Zagreb topological index Zg1 introduced by I. Gutman and N. Trinajstić in 1972 as  
Also, we know another definition of the first Zagreb index as the sum of the squares of the degrees of all vertices of G.
where du denotes the degree of u. Mathematical properties of the first Zagreb index for general graphs can be found in  -  .
Let x,yÎV(G), then the distance d(x,y) between x and y is defined as the length of any shortest path in G connecting x and y  -  .
In other words,
The radius and diameter of a graph G are defined as the minimum and maximum eccentricity among vertices of G, respectively. In other words,
Recently in 2012, M. Ghorbani and M. A. Hosseinzadeh introduced a new version of first Zagreb index (the Eccentric version and ecc(v) denotes the eccentricity of vertex v) as follows  :
In this study, we call this eccentric version of the first Zagreb index by the third Zagreb index and denote by. And in continue, a formula of the third Zagreb index for an infinite family of linear Polycene parallelogram of benzenoid by using the Cut Method is obtained.
2. Results and Discussion
In this sections, we compute the third Zagreb index M3(G) for linear Polycene parallelogram of benzenoid P(n,n) ("n ≥ 1). This family of benzenoid graph has 2n(n+2) vertices/atoms and
edges (bonds)  -  . The general representation of linear Po-
lycene parallelogram of benzenoid P(n,n) is shown in Figure 1.
Now, we can exhibit the closed formula of the third Zagreb index M3(Hk) in the following theorem.
Theorem 1. Considering the linear Polycene parallelogram of benzenoid P(n,n) ("nÎℕ), then its third Zagreb index is equal to
Proof. "nÎℕ, let P(n,n) be the linear Polycene parallelogram of benzenoid, as shown in Figure 1. To achieve our aims, we use of the Cut Method. Definition of the Cut Method and some of its properties are presented in  . Thus, we encourage readers to look at Figure 1 and see all cuts of the linear Polycene parallelogram of benzenoid P(n,n).
So according to Figure 1, one can see that the eccentric vertices with degree two are between 2n+1, 2n+2, , 4n−6, 4n−4, 4n−2, 4n−1 or the number set
And also, the eccentric vertices with degree two are between 2n, 2n+1 to 4n−4, 4n−3 or in the number set
Figure 1. The eccentric of vertices of linear polycene parallelogram of benzenoid P(n,n)  .
Therefore, by using above results and  -  , we have the following computations for the third Zagreb index of the linear Polycene parallelogram of benzenoid P(n,n) as:
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