﻿ Symmetric Division Deg Energy of a Graph
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### Symmetric Division Deg Energy of a Graph

K. N. Prakasha, P. Siva Kota Reddy, Ismail Naci Cangul
Turkish Journal of Analysis and Number Theory. 2017, 5(6), 202-209. DOI: 10.12691/tjant-5-6-2
Received July 28, 2017; Revised August 29, 2017; Accepted September 22, 2017

### Abstract

The purpose of this paper is to introduce and investigate the symmetric division deg energy SDDE(G) of a graph. We establish upper and lower bounds for SDDE(G). Also the symmetric division deg energy for certain graphs with one edge deleted are calculated.

### 1. Introduction

Let be a simple graph and let be the set of its vertices. Let If two vertices and of are adjacent, then we use the notation For a vertex the degree of will be denoted by or briefly by

In mathematical chemistry, topological indices play an important role due to their countless applications. There are many topological indices such as Randić index, sum-connectivity index, atom bond connectivity index, Zagreb indices, etc. One of those numerical descriptors, the symmetric division deg index, is included in the list of 148 discrete Adriatic indices and is a very good predictor of total surface area of polychlorobiphenyls (PCB).

The symmetric division deg index of a graph G is defined by

The concept of the symmetric division deg index motivates one to associate a symmetric square matrix SDD(G) to a graph G. The symmetric division deg matrix is, by this reason, defined as

### 2. The Symmetric Division Deg Energy of a Graph

Let G be a simple, finite, undirected graph. The classical energy E(G) is defined as the sum of the absolute values of the eigenvalues of its adjacency matrix. For more details on energy of a graph, see 2, 3.

Let SDD(G) be the symmetric division deg matrix. The characteristic polynomial of SDD(G) will be denoted by and defined as

Since the symmetric division deg matrix is real and symmetric, its eigenvalues are real numbers and we label them in non-increasing order The symmetric division deg energy of is similarly defined by

 (1)

This paper is organized as follows. In Section 3, we give some basic properties of symmetric division deg energy of a graph. In Section 4, symmetric division deg energy of some specific graphs are obtained. In Section 5, we find symmetric division deg energy of some complements of some specific graphs. In Section 6, the symmetric division deg energy for certain graphs with one edge deleted are calculated and finally in Section 7, some open problems are given.

### 3. Some Basic Properties of Symmetric Division Deg Energy of a Graph

Let us define the number P as

Then we have

Proposition 3.1. The first three coefficients of the polynomial are as follows:

(i)

(ii)

(iii)

Proof. (i) By the definition of the polynomial

we get a0 = 1.

(ii) The sum of determinants of all principal submatrices of is equal to the trace of implying that

(iii) By the definition, we have

Proposition 3.2. If are the symmetric division deg eigenvalues of , then

Proof. It follows as

Using this result, we now obtain lower and upper bounds for the symmetric division deg energy of a graph:

Theorem 3.3. Let G be a graph with n vertices. Then

Proof. Let be the eigenvalues of By the Cauchy-Schwartz inequality we have

Let Then

implying that

and hence we get

as an upper bound.

Theorem 3.4. Let G be a graph with n vertices. If then

Proof. By definition, we have

Using arithmetic-geometric mean inequality, we have

Therefore,

Thus,

Let and are the minimum and maximum values of all Then the following results can easily be proven by means of the above results:

Theorem 3.5. For a graph G of order n,

Theorem 3.6. For a graph G of order n with non-zero eigenvalues, we have

Theorem 3.7. Let G be a graph of order n. Let be the eigenvalues in increasing order. Then

### 4. Symmetric Division Deg Energy of Some Graph Types

In this section, we calculate the symmetric division deg energy of some well-known and frequently used graph types including complete, cycle, star, friendship, cocktail party, double star, Dutch windmill, crown and complete bipartite graphs.

Theorem 4.1. The symmetric division deg energy of a complete graph is

Proof. Let be the complete graph with vertex set For this graph, the symmetric division deg matrix is

The characteristic equation then becomes

and the spectrum would be

Therefore,

Theorem 4.2. The symmetric division deg energy of the cycle graph is

Proof. The symmetric division deg matrix corresponding to the cycle graph is

This is a circullant matrix of order 2n. Its eigenvalues are

Therefore the symmetric division deg energy is

Theorem 4.3. The symmetric division deg energy of the star graph is

Proof. Let be the star graph with vertex set with denotes the central vertex. The symmetric division deg matrix is

The characteristic equation becomes

and therefore, the spectrum would have an and times 0. Therefore,

Definition 4.4. The friendship graph, denoted by is defined as the graph obtained by taking n copies of the cycle graph with a vertex in common.

It is clear that

Theorem 4.5. The symmetric division deg energy of the friendship graph is

Proof. Let be the friendship graph with 2n + 1 vertices and let v0 be the common vertex. The symmetric division deg matrix is

The characteristic equation becomes

implying that the spectrum has n times , times 2, a and a Therefore, we get

Theorem 4.6. The symmetric division deg energy of the cocktail party graph is

Proof. Let be the cocktail party graph of order 2n having vertex set The symmetric division deg matrix is

In that case, the characteristic equation is

and hence the spectrum becomes

Therefore we arrive at the required result:

Theorem 4.7. The symmetric division deg energy of the double star graph is

Proof. The symmetric division deg matrix is

Hence, the spectrum would have times and Therefore, we get

Definition 4.8. A graph obtained by joining n copies of the cycle graph of length 4 at a common vertex is called a Dutch windmill graph. It will be denoted by

It is clear that the Dutch windmill graph has 3n + 1 vertices and 4n edges.

Theorem 4.9. The symmetric division deg energy of the Dutch windmill graph is

Proof. Recall that has 3n + 1 vertices. Then the symmetric division deg matrix is

Hence the characteristic equation will be

and therefore the spectrum would have times times times 0, and Therefore, it is directly seen that

Theorem 4.10. The symmetric division deg energy of crown graph is

Proof. Let be the crown graph of order 2n and let the vertex set of this graph be

The symmetric division deg matrix of is

Therefore the characteristic equation is

implying that the spectrum has a a times 2 and times . Therefore we obtain

Theorem 4.11. The symmetric division deg energy of the complete bipartite graph of order with vertex set is

Proof. The symmetric division deg matrix of the complete bipartite graph is

Then the characteristic equation is

and therefore the spectrum has a times 0 and a Therefore, we get

### 5. Symmetric Division Deg Energy of Complements

Definition 5.1. 5 Let G be a graph and be a partition of its vertex set V. Then the k-complement of G is denoted by and obtained as follows: For all and in remove the edges between and and add the edges between the vertices of and which are not in G.

Definition 5.2. 5 Let G be a graph and be a partition of its vertex set V. Then the -complement of G is denoted by and obtained as follows: For each set in remove the edges of G joining the vertices within and add the edges of (complement of G) joining the vertices of

There is usually a nice relation between some properties of a graph and its complement. Here we investigate the relation between some special graph classes and their complements in terms of the symmetric division deg energy.

Theorem 5.3. The symmetric division deg energy of the complement of the complete graph is

Proof. Let be the complete graph with vertex set The symmetric division deg connectivity matrix of the complement of the complete graph is

Clearly, the characteristic equation is implying

Theorem 5.4. The symmetric division deg energy of the complement of the star graph is

Proof. Let be the complement of the star graph with vertex set where is the central vertex. The symmetric division deg matrix is

The corresponding characteristic equation is

and therefore the spectrum is

Therefore,

Theorem 5.5. The symmetric division deg energy of the complement of the cocktail party graph of order 2n is

Proof. Let be the cocktail party graph of order 2n having the vertex set The corresponding symmetric division deg matrix is

and the characteristic equation becomes

implying that the spectrum would be

Therefore,

View option
• Figure 1. Double star graph with its 2(i)-complement

Theorem 5.6. The symmetric division deg energy of 2(i)-complement of double star graph is

where and

Proof. The symmetric division deg matrix for 2(i)-complement of double star graph is

Therefore the spectrum has times a a a and a Therefore we obtain the required result.

Theorem 5.7. The symmetric division deg energy of 2-complement of cocktail party graph is

Proof. Consider the 2-complement of the cocktail party graph The symmetric division deg matrix is

The characteristic polynomial is

and therefore the symmetric division deg spectrum has times times 0, a and a implying that the symmetric division deg energy is

### 6. Symmetric Division Deg Energy of Graphs with One Edge Deleted

Edge deletion is very important in combinatorial calculations with graphs. In this section, we obtain the symmetric division deg energy for certain graphs with one edge deleted. This can be used recursively to calculate the symmetric division deg energy of a given graph.

Theorem 6.1. Let e be an edge of the complete graph Then is equal to

Proof. The symmetric division deg matrix for is

Therefore the spectrum would have a

a times and a 0, implying the result.

Theorem 6.2. Let e be an edge of the complete bipartite graph The symmetric division deg energy of is equal to

Proof. The symmetric division deg matrix for is

Hence, the spectrum would have a a a a and times 0 implying the result

The following result can easily be proven as above:

Lemma 6.3. Let be the star graph with n vertices and let e be an edge of it. Then for

### 7. Some Open Problems

Open problem 7.1. With respect to symmetric division deg, determine the class of graphs which are co-spectral and characterize them.

Open problem 7.2. With respect to symmetric division deg, determine the class of graphs which are hyperenergetic and characterize them.

Open problem 7.3. With respect to symmetric division deg, determine the class of graphs whose symmetric division deg energy and symmetric division deg energy of their complements are equal.

Open problem 7.4. With respect to symmetric division deg, determine the class of non-co-spectral graphs which are equienergetic.

Open problem 7.5. Determine the class of graphs whose symmetric division deg energy is equal to usual energy.

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Published with license by Science and Education Publishing, Copyright © 2017 K. N. Prakasha, P. Siva Kota Reddy and Ismail Naci Cangul