On the Harary index of graph operations

Abstract

The Harary index is defined as the sum of reciprocals of distances between all pairs of vertices of a connected graph. In this paper, expressions for the Harary indices of the join, corona product, Cartesian product, composition and disjunction of graphs are derived and the indices for some well-known graphs are evaluated. In derivations some terms appear which are similar to the Harary index and we name them the second and third Harary index.

MSC:05C05, 05C07, 05C90.

1 Introduction and preliminaries

Throughout this paper we consider simple connected graphs without loops and multiple edges. Suppose that G is a graph with vertex set $V(G)=\{v_1,v_2,\dots ,v_n\}$ and edge set $E(G)$. The distance between the vertices $v_i$ and $v_j$ of $V(G)$ is denoted by $d_G(v_i,v_j)$ and it is defined as the number of edges in a minimal path connecting the vertices $v_i$ and $v_j$. The Harary index is one of very much studied topological indices and is defined as follows [1, 2]:

$$H(G)=\sum _{v_i,v_j\in V(G),i\ne j}\frac{1}{d_G(v_i,v_j)},$$

where the summation goes over all unordered pairs of vertices of G. Mathematical properties and applications of H are reported in [3–11]. We now propose two more members of the class of Harary indices, the second Harary index and the third Harary index, which are as follows:

$$H_1(G)=\sum _{v_i,v_j\in V(G),i\ne j}\frac{1}{d_G(v_i,v_j)+1}$$

and

$$H_2(G)=\sum _{v_i,v_j\in V(G),i\ne j}\frac{1}{d_G(v_i,v_j)+2},$$

where the summation goes over all unordered pairs of the vertices of G. The Wiener index of G is defined as [12]

$$W=W(G)=\sum _{\{v_i,v_j\}\subseteq V}{d}_G(v_i,v_j).$$

If we denote by $d(G,k)$ the number of vertex pairs of G, the distance of which is equal to k, then the Wiener index of G can be expressed as

$$W(G)=\sum _{k\ge 1}kd(G,k).$$

The maximum value of k, for which $d(G,k)$ is non-zero, is the diameter of the graph G, and it will be denoted by $D(G)$. The Wiener index is of certain importance in chemistry [13]. It is one of the oldest and most thoroughly studied graph-based molecular structure-descriptors (the so-called topological indices) [12–14]. Numerous chemical applications of it have been reported (see, for instance, [15, 16]), and its mathematical properties are reasonably well understood [17–20]. The degree of $v_i\in V(G)$, denoted by $d_G(v_i)$, is the number of vertices in G adjacent to $v_i$. For other undefined notations and terminology from graph theory, the readers are referred to [21].

In [22], Khalifeh et al. computed some exact formulae for the hyper-Wiener index of the join, Cartesian product, composition, disjunction and symmetric difference of graphs. Some more properties and applications of graph products can be seen in the classical book [23].

The paper is organized as follows. In Section 2, we obtain lower and upper bounds on the Harary index of graphs. In Section 3, we give some exact expressions for the Harary index of various graph operations, such as join, corona product, Cartesian product, composition, disjunction, etc. Moreover, computations are done for some well-known graphs.

2 Bounds on the Harary index

We define

$$H_t(G)=\sum _{u_i,u_k\in V(G),i\ne k}\frac{1}{d_G(u_i,u_k)+t},$$

where t is any non-negative real number. In this section we obtain lower and upper bounds on $H_t(G)$ of the graph G. From that we can find lower and upper bounds on the Harary index of graphs. These results are useful in the next section. We begin with the following lower and upper bounds on $H_t(G)$.

Theorem 1LetG ($\ncong K_{|G|}$) be a connected graph of order$|G|$, $\parallel G\parallel$edges and diameter$D(G)$. Then

$$H_t(G)\ge \frac{\parallel G\parallel }{t+1}+\frac{{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )}^2}{W(G)+\frac{|G|(|G|-1)}{2}t-(t+1)\parallel G\parallel }$$

(1)

with equality holding if and only ifGis isomorphic to a graph of diameter 2. Moreover,

$$H_t(G)\le \frac{\parallel G\parallel }{t+1}+\frac{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )[2+(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)\times (\frac{D(G)+t}{t+2}+\frac{t+2}{D(G)+t})]}{2(W(G)+\frac{|G|(|G|-1)}{2}t-(t+1)\parallel G\parallel )}$$

(2)

with equality holding if and only ifGis isomorphic to a graph of diameter 2.

Proof For $(u_i,u_k)\ne (u_j,u_{\ell })$, $i\ne k$, $j\ne \ell$, $2\le d_G(u_i,u_k)\le D(G)$, $2\le d_G(u_j,u_{\ell })\le D(G)$, we have

$$2\le (\frac{d_G(u_i,u_k)+t}{d_G(u_j,u_{\ell })+t}+\frac{d_G(u_j,u_{\ell })+t}{d_G(u_i,u_k)+t})\le (\frac{D(G)+t}{t+2}+\frac{t+2}{D(G)+t}).$$

(3)

From the definition of the Wiener index, we have

$$W(G)=\sum _{u_i,u_k\in V(G),i\ne k}{d}_G(u_i,u_k).$$

Using the above, we get

$$\sum _{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}(d_G(u_i,u_k)+t)=W(G)+\frac{|G|(|G|-1)}{2}t-(t+1)\parallel G\parallel .$$

(4)

Now,

$$\begin{array}{l}{\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}(d_G(u_i,u_k)+t){\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}\frac{1}{d_G(u_i,u_k)+t}\\ =(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )\\ +{\sum }_{(u_i,u_k)\ne (u_j,u_{\ell }),i\ne k,j\ne \ell ,d_G(u_i,u_k)\ge 2,d_G(u_j,u_{\ell })\ge 2}(\frac{d_G(u_i,u_k)+t}{d_G(u_j,u_{\ell })+t}+\frac{d_G(u_j,u_{\ell })+t}{d_G(u_i,u_k)+t}).\end{array}$$

(5)

Using (3) in the above, we get

$$\begin{array}{l}{\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}(d_G(u_i,u_k)+t){\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}\frac{1}{d_G(u_i,u_k)+t}\\ \ge (\frac{|G|(|G|-1)}{2}-\parallel G\parallel )+(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)\text{by (3)}\\ ={(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )}^2,\end{array}$$

(6)

and

$$\begin{array}{l}{\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}(d_G(u_i,u_k)+t){\sum }_{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}\frac{1}{d_G(u_i,u_k)+t}\\ \le (\frac{|G|(|G|-1)}{2}-\parallel G\parallel )+\frac{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)}{2}\\ \times (\frac{D(G)+t}{t+2}+\frac{t+2}{D(G)+t})\text{by (3)}.\end{array}$$

(7)

Now,

$$\begin{array}{rl}{H}_t(G)& =\sum _{u_i,u_k\in V(G),i\ne k}\frac{1}{d_G(u_i,u_k)+t}\\ =\frac{\parallel G\parallel }{t+1}+\sum _{u_i,u_k\in V(G),i\ne k,d_G(u_i,u_k)\ge 2}\frac{1}{d_G(u_i,u_k)+t}.\end{array}$$

Using (4), (6) and (7) in the above, we get the lower bound in (1) and upper bound in (2) on $H_t(G)$ of the graph G.

Now suppose that the equality holds in (1) and (2). Then all inequalities in the above argument must be equalities. For the lower bound, we must have

$$d_G(u_i,u_k)=2\text{for any pair of vertices }(u_i,u_k)\text{ in }V(G).$$

For the upper bound, we must have

$$d_G(u_i,u_k)=2=D(G)\text{for any pair of vertices }(u_i,u_k)\text{ in }V(G).$$

Thus G is isomorphic to a graph of diameter 2.

Conversely, one can see easily that both equalities hold in (1) and (2) for graphs of diameter 2. □

Corollary 1LetG ($\ncong K_{|G|}$) be a connected graph of order$|G|$, $\parallel G\parallel$edges and diameter$D(G)$. Then

$$H_1(G)\ge \frac{\parallel G\parallel }{2}+\frac{{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )}^2}{W(G)-2\parallel G\parallel +\frac{|G|(|G|-1)}{2}}$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2. Moreover,

$$H_1(G)\le \frac{\parallel G\parallel }{2}+\frac{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )[2+(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)\times (\frac{D(G)+1}{3}+\frac{3}{D(G)+1})]}{2(W(G)+\frac{|G|(|G|-1)}{2}-2\parallel G\parallel )}$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2.

Proof Putting $t=1$ in (1) and (2), we get the required result. By Theorem 1, we have that both equalities hold if and only if G is isomorphic to a graph of diameter 2. □

Corollary 2LetG ($\ncong K_{|G|}$) be a connected graph of order$|G|$, $\parallel G\parallel$edges and diameter$D(G)$. Then

$$H_2(G)\ge \frac{\parallel G\parallel }{3}+\frac{{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )}^2}{W(G)-3\parallel G\parallel +|G|(|G|-1)}$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2. Moreover,

$$H_2(G)\le \frac{\parallel G\parallel }{3}+\frac{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )[2+(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)\times (\frac{D(G)+2}{4}+\frac{4}{D(G)+2})]}{2(W(G)+|G|(|G|-1)-3\parallel G\parallel )}$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2.

Proof Putting $t=2$ in (1) and (2), we get the required result. By Theorem 1, we have that both equalities hold if and only if G is isomorphic to a graph of diameter 2. □

From the above Theorem 1, we get the lower and upper bounds on the Harary index of graphs.

Theorem 2LetG ($\ncong K_{|G|}$) be a connected graph of order$|G|$, $\parallel G\parallel$edges and diameter$D(G)$. Then

$$H(G)\ge \parallel G\parallel +\frac{{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )}^2}{W(G)-\parallel G\parallel }$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2. Moreover,

$$H(G)\le \parallel G\parallel +\frac{(\frac{|G|(|G|-1)}{2}-\parallel G\parallel )[2+(\frac{|G|(|G|-1)}{2}-\parallel G\parallel -1)\times (\frac{D(G)}{2}+\frac{2}{D(G)})]}{2(W(G)-\parallel G\parallel )}$$

with equality holding if and only ifGis isomorphic to a graph of diameter 2.

Proof Putting $t=0$ in (1) and (2), we get the required result. By Theorem 1, we have that both equalities hold if and only if G is isomorphic to a graph of diameter 2. □

3 Harary index of graph operations

In this section, some exact formulae for the Harary index of some graph operations are presented.

Let $G_1$ and $G_2$ be two graphs with $|G_1|$ and $|G_2|$ vertices, and $\parallel G_1\parallel$ and $\parallel G_1\parallel$ edges, respectively. The join $G_1\vee G_2$ of graphs $G_1$ and $G_2$ with disjoint vertex sets $V(G_1)$ and $V(G_2)$, and edge sets $E(G_1)$ and $E(G_2)$ is the graph union $G_1\cup G_2$ together with all the edges joining $V(G_1)$ and $V(G_2)$. Thus, for example, ${\overline{K}}_p\vee {\overline{K}}_q=K_{p,q}$, the complete bipartite graph.

Theorem 3Let$G_1$and$G_2$be two graphs. Then

$$H(G_1\vee G_2)=\frac{1}{2}(|G_1||G_2|+\parallel G_1\parallel +\parallel G_2\parallel )+\frac{1}{4}(|G_1|+|G_2|)(|G_1|+|G_2|-1).$$

Proof By the definition of the Harary index, we have

$$\begin{array}{rcl}H(G_1\vee G_2)& =& \sum _{u_i,v_j\in V(G_1\vee G_2),u_i\ne v_j}\frac{1}{d_{G_1\vee G_2}(u_i,v_j)}\\ =& \sum _{u_i,v_j\in V(G_1),u_i\ne v_j}\frac{1}{d_{G_1\vee G_2}(u_i,v_j)}+\sum _{u_i,v_j\in V(G_2),u_i\ne v_j}\frac{1}{d_{G_1\vee G_2}(u_i,v_j)}\\ +\sum _{u_i\in V(G_1)}\sum _{v_j\in V(G_2)}\frac{1}{d_{G_1\vee G_2}(u_i,v_j)}\\ =& \frac{1}{2}\sum _{u_i\in V(G_1)}(d_{G_1}(u_i)+\frac{|G_1|-d_{G_1}(u_i)-1}{2})\\ +\frac{1}{2}\sum _{v_j\in V(G_2)}(d_{G_2}(v_j)+\frac{|G_2|-d_{G_2}(v_j)-1}{2})+|G_1||G_2|\\ \text{as }{d}_{G_1\vee G_2}(u_i,v_j)=\{\begin{array}{cc}0\hfill & \text{if }{u}_i=v_j,\hfill \\ 1\hfill & \text{if }{u}_i{v}_j\in E(G_1\vee G_2),\hfill \\ 2\hfill & \text{otherwise}\hfill \end{array}\\ =& \frac{1}{2}(|G_1||G_2|+\parallel G_1\parallel +\parallel G_2\parallel )+\frac{1}{4}(|G_1|+|G_2|)(|G_1|+|G_2|-1)\\ \text{as }\sum _{u_i\in V(G_1)}{d}_{G_1}(u_i)=2\parallel G_1\parallel \text{ and }\sum _{v_j\in V(G_2)}{d}_{G_2}(v_j)=2\parallel G_2\parallel .\end{array}$$

 □

For two cycles $C_m$ and $C_n$, we have the following.

Example 1

$$\begin{array}{rcl}H(C_m\vee C_n)& =& \frac{1}{2}(mn+m+n)+\frac{1}{4}(m+n)(m+n-1)\\ =& \frac{1}{4}(m^2+n^2)+mn+\frac{1}{4}(m+n).\end{array}$$

The corona product $G_1\circ G_2$ of two graphs $G_1$ and $G_2$ is defined to be the graph Γ obtained by taking one copy of $G_1$ (which has $|G_1|$ vertices) and $|G_1|$ copies of $G_2$, and then joining the i th vertex of $G_1$ to every vertex in the i th copy of $G_2$, $i=1,2,\dots ,|G_1|$. Let $G_1=(V,E)$ and $G_2=(V,E)$ be two graphs such that $V(G_1)=\{u_1,u_2,\dots ,u_{p_1}\}$, $p_1=|G_1|$, $|E(G_1)|=\parallel G_1\parallel$ and $V(G_2)=\{v_1,v_2,\dots ,v_{p_2}\}$, $p_2=|G_2|$, $|E(G_2)|=\parallel G_2\parallel$. Then it follows from the definition of the corona that $G_1\circ G_2$ has $|G_1|(1+|G_2|)$ vertices and $\parallel G_1\parallel +|G_1|\parallel G_2\parallel +|G_1||G_2|$ edges, where

$$\begin{array}{c}V(G_1\circ G_2)=\{(u_i,v_j),i=1,2,\dots ,|G_1|;j=0,1,2\dots ,|G_2|\}\text{and}\hfill \\ E(G_1\circ G_2)=\{((u_i,v_0),(u_k,v_0)),(u_i,u_k)\in E(G_1)\}\hfill \\ \phantom{E(G_1\circ G_2)=}\cup \{((u_i,v_j),(u_i,v_{\ell })),(v_j,v_{\ell })\in E(G_2),i=1,2,\dots ,|G_1|\}\hfill \\ \phantom{E(G_1\circ G_2)=}\cup \{((u_i,v_0),(u_i,v_{\ell })),\ell =1,2,\dots ,|G_2|,i=1,2,\dots ,|G_1|\}.\hfill \end{array}$$

It is clear that if $G_1$ is connected, then $G_1\circ G_2$ is connected, and in general $G_1\circ G_2$ is not isomorphic to $G_2\circ G_1$.

Theorem 4The Harary index of the corona product is computed as follows:

$$H(G_1\circ G_2)=H(G_1)+|G_2|H_1(G_1)+{|G_2|}^2{H}_2(G_1)+\frac{1}{4}(|G_2|+3)|G_1||G_2|+\frac{1}{2}|G_1|\parallel G_2\parallel ,$$

where$H_1(G_1)={\sum }_{u_i,u_k\in V(G_1),u_i\ne u_k}\frac{1}{d_{G_1}(u_i,u_k)+1}$and$H_2(G_1)={\sum }_{u_i,u_k\in V(G_1),u_i\ne u_k}\frac{1}{d_{G_1}(u_i,u_k)+2}$.

Proof Note that

$$H_k(G_1)=\sum _{u_i,u_j\in V(G_1),u_i\ne u_j}\frac{1}{d_{G_1}(u_i,u_j)+k}\text{for }k=1,2.$$

By the definition of the Harary index, we have

$$\begin{array}{rcl}H(G_1\circ G_2)& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\circ G_2),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{G_1\circ G_2}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{(u_i,v_0),(u_k,v_0)\in V(G_1\circ G_2),i\ne k}\frac{1}{d_{G_1\circ G_2}((u_i,v_0),(u_k,v_0))}\\ +\sum _{(u_i,v_0),(u_k,v_{\ell })\in V(G_1\circ G_2),\ell \ne 0}\frac{1}{d_{G_1\circ G_2}((u_i,v_0),(u_k,v_{\ell }))}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\circ G_2),(u_i,v_j)\ne (u_k,v_{\ell }),j\ne 0\ne \ell }\frac{1}{d_{G_1\circ G_2}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)}\\ +\sum _{(u_i,v_0),(u_i,v_{\ell })\in V(G_1\circ G_2),\ell \ne 0}\frac{1}{d_{G_1\circ G_2}((u_i,v_0),(u_i,v_{\ell }))}\\ +\sum _{(u_i,v_0),(u_k,v_{\ell })\in V(G_1\circ G_2),\ell \ne 0,i\ne k}\frac{1}{d_{G_1\circ G_2}((u_i,v_0),(u_k,v_{\ell }))}\\ +\sum _{(u_i,v_j),(u_i,v_{\ell })\in V(G_1\circ G_2),0\ne j\ne \ell \ne 0}\frac{1}{d_{G_1\circ G_2}((u_i,v_j),(u_i,v_{\ell }))}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\circ G_2),j\ne 0\ne \ell ,i\ne k}\frac{1}{d_{G_1\circ G_2}((u_i,v_j),(u_k,v_{\ell }))}\\ =& H(G_1)+\sum _{u_i\in V(G_1)}|G_2|+\sum _{v_j\in V(G_2)}\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)+1}\\ +\sum _{u_i\in V(G_1)}\frac{1}{2}\sum _{j=1}^{|G_2|}[d_{G_2}(v_j)+\frac{1}{2}(|G_2|-d_{G_2}(v_j)-1)]\\ +\sum _{v_j\in V(G_2)}\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{|G_2|}{d_{G_1}(u_i,u_k)+2}\\ =& H(G_1)+|G_1||G_2|+|G_2|\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)+1}\\ +\frac{1}{4}|G_1|\sum _{j=1}^{|G_2|}(|G_2|+d_{G_2}(v_j)-1)+{|G_2|}^2\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)+2}\\ =& H(G_1)+|G_2|H_1(G_1)+{|G_2|}^2{H}_2(G_1)\\ +\frac{1}{4}(|G_2|+3)|G_1||G_2|+\frac{1}{2}|G_1|\parallel G_2\parallel .\end{array}$$

 □

Theorem 5Let$G_1$ ($\ncong K_{|G_1|}$) and$G_2$be two connected graphs with diameter of$G_1$, $D(G_1)$. Then the lower and upper bounds on the Harary index of the corona product are as follows:

$$\begin{array}{c}H(G_1\circ G_2)\hfill \\ \ge [\frac{1}{W(G_1)-\parallel G_1\parallel }+\frac{|G_2|}{W(G_1)-2\parallel G_1\parallel +\frac{|G_1|(|G_1|-1)}{2}}\hfill \\ +\frac{{|G_2|}^2}{W(G_1)-3\parallel G_1\parallel +|G_1|(|G_1|-1)}]\hfill \\ \times {(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )}^2+\parallel G_1\parallel (1+\frac{|G_2|}{2}+\frac{{|G_2|}^2}{3})\hfill \\ +\frac{1}{4}(|G_2|+3)|G_1||G_2|+\frac{1}{2}|G_1|\parallel G_2\parallel \hfill \end{array}$$

with equality holding if and only if$G_1$is isomorphic to a graph of diameter 2. Moreover,

$$\begin{array}{c}H(G_1\circ G_2)\hfill \\ \le (\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )\times [\frac{2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel -1)(\frac{D(G_1)}{2}+\frac{2}{D(G_1)})}{2(W(G_1)-\parallel G_1\parallel )}\hfill \\ +\frac{|G_2|(2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel -1)(\frac{D(G_1)+1}{3}+\frac{3}{D(G_1)+1}))}{2(W(G_1)-2\parallel G_1\parallel +\frac{|G_1|(|G_1|-1)}{2})}\hfill \\ +\frac{{|G_2|}^2(2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel -1)(\frac{D(G_1)+2}{4}+\frac{4}{D(G_1)+2}))}{2(W(G_1)-3\parallel G_1\parallel +|G_1|(|G_1|-1))}]\hfill \\ +\parallel G_1\parallel (1+\frac{|G_2|}{2}+\frac{{|G_2|}^2}{3})+\frac{1}{4}(|G_2|+3)|G_1||G_2|+\frac{1}{2}|G_1|\parallel G_2\parallel \hfill \end{array}$$

with equality holding if and only if$G_1$is isomorphic to a graph of diameter 2.

Proof Using Corollaries 1 and 2 with Theorem 2, from Theorem 4, we get the lower and upper bounds on the Harary index of the corona product. Moreover, the equality holds if and only if $G_1$ is isomorphic to a graph of diameter 2. □

Given a graph G with vertex set $V=\{v_1,v_2,\dots ,v_n\}$, the thorn graph $G(p_1,p_2,\dots ,p_n)$ first introduced by Gutman [24], is a graph obtained by attaching $p_i$ pendent vertices to vertex $v_i$ for $i=1,2,\dots ,n$. In particular, if $p_1=p_2=\cdots =p_n=p$, we denote by $G^{(p)}$ the thorn graph $G(p_1,p_2,\dots ,p_n)$ for short. Recall the definition of the corona product, the graph $G^{(p)}\cong G\circ \overline{K_p}$, where $\overline{K_p}$ denotes the complement of a complete graph $K_p$. Therefore, for a connected graph G of order n, we have the following.

Example 2

$$H\left(G^{(p)}\right)=H(G\circ \overline{K_p})=H(G)+p{H}_1(G)+p^2{H}_2(G_1)+\frac{1}{4}(p+3)np.$$

The Cartesian product $G_1\times G_2$ of graphs $G_1$ and $G_2$ has the vertex set $V(G_1\times G_2)=V(G_1)\times V(G_2)$ and $(u_i,v_j)(u_k,v_{\ell })$ is an edge of $G_1\times G_2$ if $u_i=u_k$ and $v_j{v}_{\ell }\in E(G_2)$, or $u_i{u}_k\in E(G_1)$ and $v_j=v_{\ell }$.

Theorem 6Let$G_1$and$G_2$be two connected graphs with diameter$D(G_2)$of the graph$G_2$. Then

$$\begin{array}{l}|G_1|H(G_2)+|G_2|H(G_1)+|G_2|(|G_2|-1)H_D(G_1)\\ \le H(G_1\times G_2)\\ \le |G_1|H(G_2)+|G_2|H(G_1)+|G_2|(|G_2|-1)H_1(G_1),\end{array}$$

(8)

where

$$\begin{array}{r}{H}_D(G_1)=\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)+D(G_2)}\mathit{\text{and}}\\ H_1(G_1)=\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)+1}.\end{array}$$

Moreover, both sides of the equality hold in (8) if and only if$G_2$is isomorphic to a complete graph of order$|G_2|$.

Proof By the definition of the Harary index, we have

$$\begin{array}{rcl}H(G_1\times G_2)& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\times G_2),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{G_1\times G_2}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\times G_2),i=k,j\ne \ell }\frac{1}{d_{G_1}(u_i,u_k)+d_{G_2}(v_j,v_{\ell })}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\times G_2),i\ne k,j=\ell }\frac{1}{d_{G_1}(u_i,u_k)+d_{G_2}(v_j,v_{\ell })}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\times G_2),i\ne k,j\ne \ell }\frac{1}{d_{G_1}(u_i,u_k)+d_{G_2}(v_j,v_{\ell })}\\ =& \sum _{u_i\in V(G_1)}\sum _{v_j,v_{\ell }\in V(G_2),j\ne \ell }\frac{1}{d_{G_2}(v_j,v_{\ell })}\\ +\sum _{u_i,u_k\in V(G_1),i\ne k}\sum _{v_j\in V(G_2)}\frac{1}{d_{G_1}(u_i,u_k)}\\ +\sum _{u_i,u_k\in V(G_1),i\ne k}\sum _{v_j,v_{\ell }\in V(G_2),j\ne \ell }\frac{1}{d_{G_1}(u_i,u_k)+d_{G_2}(v_j,v_{\ell })}\\ \text{as }{d}_{G_1}(u_i,u_i)=0\text{ and }{d}_{G_2}(v_j,v_j)=0\\ \le & |G_1|H(G_2)+|G_2|H(G_1)\\ +\sum _{(u_i,u_k)\in V(G_1)}\sum _{(v_j,v_{\ell })\in V(G_2)}\frac{1}{d_{G_1}(u_i,u_k)+1}\\ \text{as }{d}_{G_2}(v_j,v_{\ell })\ge 1,j\ne \ell \\ =& |G_1|H(G_2)+|G_2|H(G_1)+|G_2|(|G_2|-1)H_1(G_1),\end{array}$$

(9)

where $H_1(G_1)$ is given in the statement of the theorem. Since $d_{G_2}(v_j,v_{\ell })\le D(G_2)$, similarly, we get

$$\begin{array}{rcl}H(G_1\times G_2)& \ge & |G_1|H(G_2)+|G_2|H(G_1)\\ +\sum _{(u_i,u_k)\in V(G_1)}\sum _{(v_j,v_{\ell })\in V(G_2)}\frac{1}{d_{G_1}(u_i,u_k)+D(G_2)}\\ =& |G_1|H(G_2)+|G_2|H(G_1)+|G_2|(|G_2|-1)H_D(G_1),\end{array}$$

where $H_D(G_1)$ is given in the statement of the theorem. The first part of the proof is over.

Suppose that both sides of the equality hold in (8). Then we must have $d_{G_2}(v_j,v_{\ell })=1$, $v_j\ne v_{\ell }$, for all $v_j,v_{\ell }\in V(G_2)$ or $d_{G_2}(v_j,v_{\ell })=D(G_2)$, $v_j\ne v_{\ell }$, for all $v_j,v_{\ell }\in V(G_2)$. Hence $G_2$ is isomorphic to a complete graph of order $|G_2|$.

Conversely, one can see easily that (8) holds for $G_2$, a complete graph of order $|G_2|$. This completes the proof. □

Theorem 7Let$G_1$ ($\ncong K_{|G_1|}$) and$G_2$ ($\ncong K_{|G_2|}$) be two connected graphs with diameter$D(G_1)$of the graph$G_1$and$D(G_2)$of the graph$G_2$. Then

$$\begin{array}{rcl}H(G_1\times G_2)& >& |G_1|\parallel G_2\parallel +|G_2|\parallel G_1\parallel +\frac{|G_2|(|G_2|-1)\parallel G_1\parallel }{D(G_2)+1}+\frac{{(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel )}^2|G_1|}{W(G_2)-\parallel G_2\parallel }\\ +[\frac{|G_2|}{W(G_1)-\parallel G_1\parallel }+\frac{|G_2|(|G_2|-1)}{W(G_1)+\frac{|G_1|(|G_1|-1)}{2}D(G_2)-(D(G_2)+1)\parallel G_1\parallel }]\\ \times {(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )}^2.\end{array}$$

Moreover,

$$\begin{array}{rcl}H(G_1\times G_2)& <& |G_1|\parallel G_2\parallel +|G_2|\parallel G_1\parallel +\frac{|G_2|(|G_2|-1)\parallel G_1\parallel }{2}\\ +\frac{|G_1|(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel )[2+(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel )\times (\frac{D(G_2)}{2}+\frac{2}{D(G_2)})]}{2(W(G_2)-\parallel G_2\parallel )}\\ +[\frac{|G_2|(2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel -1)(\frac{D(G_1)}{2}+\frac{2}{D(G_1)}))}{2(W(G_1)-\parallel G_1\parallel )}\\ +\frac{|G_2|(|G_2|-1)(2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel -1)(\frac{D(G_1)+1}{3}+\frac{3}{D(G_1)+1}))}{2(W(G_1)+\frac{|G_1|(|G_1|-1)}{2}-2\parallel G_1\parallel )}]\\ \times (\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel ).\end{array}$$

Proof Using Theorems 1 and 2 with Corollary 1 in Theorem 6, we get the lower and upper bounds on the Cartesian product $G_1\times G_2$ of the graphs $G_1$ and $G_2$. Moreover, both inequalities are strict as $G_1\ncong K_{|G_1|}$, $G_2\ncong K_{|G_2|}$ and by Theorem 6. □

Theorem 8Let$G_1$and$G_2$be two connected graphs with diameter$D(G_1)$of the graph$G_1$. Then

$$\begin{array}{l}|G_1|H(G_2)+|G_2|H(G_1)+|G_1|(|G_1|-1)H_D(G_2)\\ \le H(G_1\times G_2)\\ \le |G_1|H(G_2)+|G_2|H(G_1)+|G_1|(|G_1|-1)H_1(G_2),\end{array}$$

(10)

where

$$\begin{array}{c}{H}_D(G_2)=\sum _{v_j,v_{\ell }\in V(G_2),j\ne \ell }\frac{1}{d_{G_2}(v_j,v_{\ell })+D(G_1)}\mathit{\text{and}}\hfill \\ H_1(G_2)=\sum _{v_j,v_{\ell }\in V(G_1),j\ne \ell }\frac{1}{d_{G_2}(v_j,v_{\ell })+1}.\hfill \end{array}$$

Moreover, both sides of the equality hold in (10) if and only if$G_1$is isomorphic to a complete graph of order$|G_1|$.

Proof Since $1\le d_{G_1}(u_i,u_k)\le D(G_1)$, $i\ne k$, from (9) we get the required result in (10). Moreover, both sides of the equality hold in (10) if and only if $G_1$ is isomorphic to a complete graph of order $|G_1|$. □

Theorem 9Let$G_1$ ($\ncong K_{|G_1|}$) and$G_2$ ($\ncong K_{|G_2|}$) be two connected graphs with diameter$D(G_1)$of the graph$G_1$and$D(G_2)$of the graph$G_2$. Then

$$\begin{array}{rcl}H(G_1\times G_2)& >& |G_1|\parallel G_2\parallel +|G_2|\parallel G_1\parallel +\frac{|G_1|(|G_1|-1)\parallel G_2\parallel }{D(G_1)+1}\\ +\frac{{(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )}^2|G_2|}{W(G_1)-\parallel G_1\parallel }\\ +[\frac{|G_1|}{W(G_2)-\parallel G_2\parallel }+\frac{|G_1|(|G_1|-1)}{W(G_2)+\frac{|G_2|(|G_2|-1)}{2}D(G_1)-(D(G_1)+1)\parallel G_2\parallel }]\\ \times {(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel )}^2.\end{array}$$

Moreover,

$$\begin{array}{rcl}H(G_1\times G_2)& <& |G_1|\parallel G_2\parallel +|G_2|\parallel G_1\parallel +\frac{|G_1|(|G_1|-1)\parallel G_2\parallel }{2}\\ +\frac{|G_2|(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )[2+(\frac{|G_1|(|G_1|-1)}{2}-\parallel G_1\parallel )\times (\frac{D(G_1)}{2}+\frac{2}{D(G_1)})]}{2(W(G_1)-\parallel G_1\parallel )}\\ +[\frac{|G_1|(2+(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel -1)(\frac{D(G_2)}{2}+\frac{2}{D(G_2)}))}{2(W(G_2)-\parallel G_2\parallel )}\\ +\frac{|G_1|(|G_1|-1)(2+(\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel -1)(\frac{D(G_2)+1}{3}+\frac{3}{D(G_2)+1}))}{2(W(G_2)+\frac{|G_2|(|G_2|-1)}{2}-2\parallel G_2\parallel )}]\\ \times (\frac{|G_2|(|G_2|-1)}{2}-\parallel G_2\parallel ).\end{array}$$

Proof Using Theorems 1 and 2 with Corollary 1 in Theorem 8, we get the lower and upper bound on the Cartesian product $G_1\times G_2$ of graphs $G_1$ and $G_2$. Moreover, both inequalities are strict as $G_1\ncong K_{|G_1|}$, $G_2\ncong K_{|G_2|}$ and by Theorem 8. □

Corollary 3LetGbe a connected graph of order$|G|$. Then

$$H(G\times K_2)=2H(G)+|G|+2H_1(G),$$

where$H_1(G)={\sum }_{v_i,v_j\in V(G),i\ne j}\frac{1}{d_G(v_i,v_j)+1}$.

Proof Choosing $G_1=G$ and $G_2=K_2$ in Theorem 6, this theorem follows immediately. □

The lattice graph $L_{2,n}$ (see [25]) is just $P_n\times K_2$. It is well known that [2]$H(P_n)=n{\sum }_{i=1}^{n-1}\frac{1}{i}-n+1$. So, we have the following example.

Example 3

$$\begin{array}{rl}H(L_{2,n})& =2H(P_n)+n+2H_1(P_n)=2n\sum _{i=1}^{n-1}\frac{1}{i}-2n+2+n+2H_1(P_n)\\ =4n\sum _{i=3}^{n-1}\frac{1}{i}+n+6.\end{array}$$

The composition (also called lexicographic product [26]) $G=G_1[G_2]$ of graphs $G_1$ and $G_2$ with disjoint vertex sets $V(G_1)$ and $V(G_2)$ and edge sets $E(G_1)$ and $E(G_2)$ is the graph with vertex set $V(G_1)\times V(G_2)$ and $(u_i,v_j)$ is adjacent with $(u_k,v_{\ell })$ whenever $u_i$ is adjacent with $u_k$, or $u_i=u_k$ and $v_j$ is adjacent with $v_{\ell }$.

Theorem 10Let$G_1$and$G_2$be two connected graphs. Then

$$H(G_1[G_2])=\frac{1}{4}|G_1||G_2|(|G_2|-1)+\frac{1}{2}|G_1|\parallel G_2\parallel +{|G_2|}^{2}H(G_1).$$

Proof By the definition of the Harary index, we have

$$\begin{array}{rcl}H(G_1[G_2])& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1[G_2]),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{G_1[G_2]}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{(u_i,v_j),(u_i,v_{\ell })\in V(G_1[G_2]),j\ne \ell }\frac{1}{d_{G_1[G_2]}((u_i,v_j),(u_i,v_{\ell }))}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1[G_2]),i\ne k}\frac{1}{d_{G_1[G_2]}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{u_i\in V(G_1)}\frac{1}{2}\sum _{j=1}^{|G_2|}[d_{G_2}(v_j)+\frac{1}{2}(|G_2|-d_{G_2}(v_j)-1)]\\ +\sum _{v_j\in V(G_2)}\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{|G_2|}{d_{G_1}(u_i,u_k)}\\ \text{as }{d}_{G_1[G_2]}((u_i,v_j),(u_i,v_{\ell }))=\{\begin{array}{cc}0\hfill & \text{if }{v}_j=v_{\ell },\hfill \\ 1\hfill & \text{if }{v}_j{v}_{\ell }\in E(G_2),\hfill \\ 2\hfill & \text{otherwise}\hfill \end{array}\\ =& \frac{|G_1|}{4}\sum _{j=1}^{|G_2|}(|G_2|+d_{G_2}(v_j)-1)+{|G_2|}^2\sum _{u_i,u_k\in V(G_1),i\ne k}\frac{1}{d_{G_1}(u_i,u_k)}\\ =& \frac{1}{4}|G_1||G_2|(|G_2|-1)+\frac{1}{2}|G_1|\parallel G_2\parallel +{|G_2|}^{2}H(G_1).\end{array}$$

 □

The double graph of a given graph G, denoted by $G^{⊛}$, is constructed by making two copies of G (including the initial edge set of each), denoted by $G_1$ and $G_2$, and adding edges $u_1{v}_2$ and $u_2{v}_1$ for every edge uv of G. From the definition of composition, we conclude that $G^{⊛}\cong G[K_2]$ for any connected graph G. Therefore the following corollary can be easily obtained.

Corollary 4LetGbe a connected graph. Then

$$H(G[K_2])=\frac{3|G|}{2}+4H(G).$$

For a complete graph $K_n$, we find that $K_n^{⊛}$ is a graph obtained by deleting a perfect matching from the complete graph $K_{2n}$, which is just the well-known cocktail party graph (see [27]).

Example 4

$$H\left(K_n^{⊛}\right)=\frac{3n}{2}+4H(K_n)=2n^2-\frac{n}{2}.$$

The disjunction $G_1\otimes G_2$ of graphs $G_1$ and $G_2$ is the graph with vertex set $V(G_1)\times V(G_2)$ and $(u_i,v_j)$ is adjacent with $(u_k,v_{\ell })$ whenever $u_i{u}_k\in E(G_1)$ or $v_j{v}_{\ell }\in E(G_2)$.

Theorem 11Let$G_1$and$G_2$be two connected graphs. Then

$$H(G_1\otimes G_2)=\frac{1}{4}|G_1||G_2|(|G_1||G_2|-1)+\frac{1}{2}(\parallel G_1\parallel {|G_2|}^2+\parallel G_2\parallel {|G_1|}^2)-\parallel G_1\parallel \parallel G_2\parallel .$$

Proof In [22], it has been proved that

$$d_{G_1\otimes G_2}((u_i,v_j),(u_k,v_{\ell }))=\{\begin{array}{cc}0\hfill & u_i=u_k\text{ and }{v}_j=v_{\ell },\hfill \\ 1\hfill & u_i{u}_k\in E(G)\text{ or }{v}_j{v}_{\ell }\in E(H),\hfill \\ 2\hfill & \text{otherwise}.\hfill \end{array}$$

(11)

Moreover, it has been showed that

$$\begin{array}{l}\left|\{v\in V(G_1\otimes G_2)|d_{G_1\otimes G_2}((u_i,v_j),v)=1\}\right|\\ =d_{G_1}(u_i)|G_2|+d_{G_2}(v_j)|G_1|-d_{G_1}(u_i)d_{G_2}(v_j).\end{array}$$

(12)

By the definition of the Harary index, we have

$$\begin{array}{rcl}H(G_1\otimes G_2)& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(G_1\otimes G_2),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{G_1\otimes G_2}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \frac{1}{2}\sum _{(u_i,v_j)\in V(G_1\otimes G_2)}[d_{G_1\otimes G_2}((u_i,v_j))+\frac{1}{2}(|G_1||G_2|-d_{G_1\otimes G_2}((u_i,v_j))-1)]\\ \text{by (11)}\\ =& \frac{1}{4}|G_1||G_2|(|G_1||G_2|-1)\\ +\frac{1}{4}\sum _{(u_i,v_j)\in V(G_1\otimes G_2)}(d_{G_1}(u_i)|G_2|+d_{G_2}(v_j)|G_1|-d_{G_1}(u_i)d_{G_2}(v_j))\text{by (12)}\\ =& \frac{1}{4}|G_1||G_2|(|G_1||G_2|-1)\\ +\frac{1}{4}\sum _{u_i\in V(G_1)}\sum _{v_j\in V(G_2)}(d_{G_1}(u_i)|G_2|+d_{G_2}(v_j)|G_1|-d_{G_1}(u_i)d_{G_2}(v_j))\\ =& \frac{1}{4}|G_1||G_2|(|G_1||G_2|-1)+\frac{1}{2}(\parallel G_1\parallel {|G_2|}^2+\parallel G_2\parallel {|G_1|}^2)-\parallel G_1\parallel \parallel G_2\parallel .\end{array}$$

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The construction of the extended double cover was introduced by Alon [28] in 1986. For a simple graph G with vertex set $V=\{v_1,v_2,\dots ,v_n\}$, the extended double cover of G, denoted by $G^{\star }$, is the bipartite graph with bipartition $(X;Y)$ where $X=\{x_1,x_2,\dots ,x_n\}$ and $Y=\{y_1,y_2,\dots ,y_n\}$, in which $x_i$ and $y_j$ are adjacent if and only if $i=j$ or $v_i$ and $v_j$ are adjacent in G. Note that for a graph G, $G^{\star }\cong G\otimes K_2$. So, the corollary below follows immediately.

Corollary 5LetGbe a connected graph of ordern. Then

$$H(G\otimes K_2)=\frac{1}{2}n(2n-1)+\frac{1}{2}(4\parallel G\parallel +n^2)-\parallel G\parallel .$$

For a complete graph $K_n$, by the definition listed above, we find that $K_n\otimes K_2$ is just $K_{n,n}$.

Example 5

$$H(K_n\otimes K_2)=\frac{1}{2}n(2n-1)+\frac{1}{2}(4\parallel K_n\parallel +n^2)-\parallel K_n\parallel =2n^2-n.$$

Let $G=(V,E)$ be a connected graph of $|G|$ vertices with $\parallel G\parallel$ edges. If we put two similar graphs G side by side, and any vertex of the first graph G is connected by edges with those vertices which are adjacent to the corresponding vertex of the second graph G and the resultant graph is denoted by $K_2\uplus G$, then we have $|K_2\uplus G|=2|G|$ and $\parallel K_2\uplus G\parallel =\parallel G\parallel +\parallel G\parallel +2\parallel G\parallel =4\parallel G\parallel$. Moreover, $K_2\uplus G$ is the graph of $K_2$ and G with the vertex set $V(K_2\times G)=V(K_2)\times V(G)$ and $(u_i,v_j)(u_k,v_{\ell })$ is an edge of $K_2\times G$ whenever ($u_i=u_k$ and $v_j$ is adjacent with $v_{\ell }$) or ($u_i\ne u_k$ and $v_j$ is adjacent with $v_{\ell }$).

Theorem 12LetGbe a connected graph. Then

$$H(K_2\uplus G)=4H(G)+\frac{|G|}{2}.$$

Proof By the definition of the Harary index, we have

$$\begin{array}{rcl}H(K_2\uplus G)& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(K_2\uplus G_2),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{K_2\uplus G}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{(u_i,v_j),(u_i,v_{\ell })\in V(K_2\uplus G),j\ne \ell }\frac{1}{d_{K_2\uplus G}((u_i,v_j),(u_i,v_{\ell }))}\\ +\sum _{(u_i,v_j),(u_k,v_{\ell })\in V(K_2\uplus G),i\ne k}\frac{1}{d_{K_2\uplus G}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{u_i\in K_2}\sum _{v_j,v_{\ell }\in V(G),j\ne \ell }\frac{1}{d_G(v_j,v_{\ell })}+\sum _{v_j\in V(G)}[\frac{1}{2}+\sum _{v_{\ell }\in V(G),\ell \ne j}\frac{1}{d_G(v_j,v_{\ell })}]\\ =& 2\sum _{v_j,v_{\ell }\in V(G),j\ne \ell }\frac{1}{d_G(v_j,v_{\ell })}+\frac{|G|}{2}+\sum _{v_j\in V(G)}\sum _{v_{\ell }\in V(G),\ell \ne j}\frac{1}{d_G(v_j,v_{\ell })}\\ =& 4H(G)+\frac{|G|}{2}\\ \text{as }H(G)=\sum _{v_j,v_{\ell }\in V(G),\ell \ne j}\frac{1}{d_G(v_j,v_{\ell })}=\frac{1}{2}\sum _{v_j\in V(G)}\sum _{v_{\ell }\in V(G),\ell \ne j}\frac{1}{d_G(v_j,v_{\ell })}.\end{array}$$

 □

Let $G=(V,E)$ be a connected graph of $|G|$ vertices with $\parallel G\parallel$ edges. If we put two similar graphs G side by side, and any vertex of the first graph G is connected by edges with those vertices which are nonadjacent to the corresponding vertex (including the corresponding vertex itself) of the second graph G and the resultant graph is denoted by $K_2\bigsqcup G$, then we have $|K_2\bigsqcup G|=2|G|$ and $\parallel K_2\bigsqcup G\parallel =\parallel G\parallel +\parallel G\parallel +{|G|}^2-2\parallel G\parallel ={|G|}^2$. Moreover, $K_2\bigsqcup G$ is the graph of $K_2$ and G with the vertex set $V(K_2\bigsqcup G)=V(K_2)\times V(G)$ and $(u_i,v_j)(u_k,v_{\ell })$ is an edge of $K_2\times G$ whenever ($u_i=u_k$ and $v_j$ is adjacent with $v_{\ell }$) or ($u_i\ne u_k$ and $v_j$ is nonadjacent with $v_{\ell }$).

Theorem 13LetGbe a connected graph of order$|G|$. Then

$$H(K_2\bigsqcup G)=\frac{|G|(3|G|-1)}{2}.$$

Proof In $K_2\bigsqcup G$, for each vertex $(u_i,v_j)$, there are $d_G(v_j)+|G|-1-d_G(v_j)+1=|G|$ neighbors, and $d_G(v_j)+n-1-d_G(v_j)=|G|-1$ vertices with the distance 2 from itself. By the definition of the Harary index, we have

$$\begin{array}{rcl}H(K_2\bigsqcup G)& =& \sum _{(u_i,v_j),(u_k,v_{\ell })\in V(K_2\bigsqcup G_2),(u_i,v_j)\ne (u_k,v_{\ell })}\frac{1}{d_{K_2\bigsqcup G}((u_i,v_j),(u_k,v_{\ell }))}\\ =& \sum _{u_i\in K_2}\frac{1}{2}\sum _{v_j\in G}(d_{K_2\bigsqcup G}(u_i,v_j)+\frac{2|G|-d_{K_2\bigsqcup G}(u_i,v_j)-1}{2})\\ =& \sum _{u_i\in K_2}\frac{1}{2}\sum _{v_j\in G}(|G|+\frac{|G|-1}{2})\\ =& \frac{|G|(3|G|-1)}{2}.\end{array}$$

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