1. Introduction and Preliminaries
In this paper, only simple, undirected, and finite graphs are considered. Assume that
is a graph with vertex set
and edge set
. The order of
G is denoted by
and the size of
G is denoted by
. The fundamental expressions and methodologies of graph theory have been used (see [
1]). Assume that
G is a graph of
n vertices,
adjacency matrix is a matrix, such that
equals 1 if vertices
and
are adjacent and
zero for otherwise. Assume that
is the vertex degree diagonal matrix of order
where
is the degree of
Then
is called the
(combinational) Laplacian matrix of
G.
The traditional concept of distance between vertices
and
, is the length of the shortest path obtained by joining these vertices of graph
G and that is denoted by
. In graph theory, distance is also an essential invariant from which distance based parameters are obtained. In [
2], a well-known distance based parameter named as
Wiener index and denoted by
is introduced for the first time. This parameter is obtained by adding the distances between every pair of vertices in
G, that is,
. Eventually, Gutman [
3] also developed the Gutman index, which is a weighted variant of the Wiener index and defined as
.
In [
4], Klein and Randić suggested a novel distance function
for a graph depending upon the electrical network theory, which is called the resistance distance. The Kirchhoff index was generated by Klein and Ivanciuc [
5] and Klein and Randi [
4] based on the resistance distance parameter and is denoted by
.
Gutman and Mohar [
6] established a result separately for the Kirchhoff index that is represented as,
where
with
and
are the eigenvalues of
.
In [
7], Chung proposed the
normalized Laplacian, denoted by
, that is,
As a result, it is easy to understand that
where
(
) be the degree of the vertex
(
) and
show the
-th values of
. In
Chen and Zhang [
8] presented a new index based on normalized Laplacian, which is called the
multiplicative degree-Kirchhoff index and defined it as,
. Moreover, the Kirchhoff index has gained much attention due to its widespread use in chemistry, physics, mathematics, and theoretical computer science. Many scientists have previously proposed new closed formulae of Kirchhoff and multiplicative degree-Kirchhoff indies, along with linear polyomino chains [
9], circulant graphs [
10], quadrilateral graphs [
11], cycles [
12], composite graphs [
13], and so on. There are several results on the degree-Kirchhoff index and normalized Laplacian (see [
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25]).
Due to the widely applications, the molecular graphs phenylenes, pentagonal, hexagonal, and octagonal networks have attracted strong interests of chemists, mathematicians, and engineering. Phenylenes are two connected graphs with structure-property that each of their cells (or interior faces) has become a hexagon or square with the same edge length. Phenylene systems are essential in theoretical chemistry because they can reflect a hydrocarbon in nature. In [
26,
27], Gutman et al. addressed the phenylene enumeration problem using the Kekulé structure. Later, Pavlović and Gutman [
28] have solved the problem of evaluating the Wiener index of phenylenes. Chen and Zhang [
29] calculated the expected value of Wiener index (or the number of perfect matches) of the unique phenylene chain using a specific analytical expression. Recently, Geng and Lei [
30] explicitly calculated the Kirchhoff index and spanning tree of phenylene chain.
In this article, by using the decomposition theorem for the normalized Laplacian characteristic polynomial, the multiplicative degree-Kirchhoff index, as well as spanning trees of are given explicit closed-form formulas.
3. Main Results
In this section, the normalized Laplacian eigenvalues of
are first obtained. Next, we resolve to discover the calculation for multiplicative degree-Kirchhoff index (or spanning tree) of
. Given a square matrix
A with order
n, we will write to
as the submatrix generated by eliminating the
columns and rows of
A. According to Equation
the block matrices could be represented as follows.
and
, a diagonal matrix of order
.
Assume that the eigenvalues of and are respectively, represented as and . According to Lemma 1 the spectrum of is and it is simple to see that and
Hence, we have the lemma below.
Lemma 4. Suppose that be the generalized phenylenes squares and n hexagons. Thenwhere , and , are eigenvalues of and , respectively. According to the relationship between the coefficients and roots of (respectively. ), standard formulae of (respectively. ) are given in the next lemmas.
Lemma 5. Suppose that to be described as above. Then Proof. Suppose that
Then
are roots satisfies the equation below:
and so
are roots satisfies the equation below:
Hence, by Vieta’s theorem (see [
31], p. 81), we obtain
For the sake of convenience, we take
to the
i-th order principal sub-matrix of
, yield by the first
columns and rows,
. Let
. Then
and
These formulas in general form could be derived by a straightforward calculation as shown below.
Case 1. Suppose
,
,
and
are defined as above. We have
In contrast, we take
to the
i-th order principal sub-matrix of
, built into the last
columns and rows,
. Let
. Then
and
These formulas in general form could be derived by a straightforward calculation as shown below.
Case 2. Suppose
,
,
and
are defined as above. We have
When case 1 and case 2 are combined, the following fact can be deduced.
Fact 1.
Proof of Fact 1. The diagonal entries of
are denoted by
. Due to the
is a total of all principal minors of
with
rows and columns, we have
where
Combining Case 1, Case 2 and Equation (
6), we find
as desired.
Fact 2..
Proof of Fact 2. Due to the
is total of all principal minors of
with
rows and columns, we have
where
In view of Equation (
8), we know that
will change as a result of the various
i and
j options. Hence, we will discuss by separating the cases below.
Case 1. Let
and
for
Therefore,
In this case,
X is the square matrix of order
Case 2. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 3. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 4. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 5. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 6. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 7. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 8. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 9. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 10. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 11. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 12. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 13. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 14. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 15. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
Case 16. Let
and
for
Therefore,
In this case,
X is the square matrix of order
.
If Equation (
8) and Cases 1–16 are combined, we have
where
By substituting
,
and
into Equation (
9), the Fact 3 is completed.
The required results of Lemma 5 can be obtained by combining Fact 1 and Fact 3. □
Lemma 6. Let be the eigenvalues of as above. Thenwhere and
Proof. Suppose that
So
are roots satisfies the equation below:
By Vieta’s theorem (see [
31],
), we find
In order to find
and
, consider
to the
i-th order principal sub-matrix of
, created by the first
i rows and columns,
. Let
. Then
,
and
We have adopted similar computation as described above.
Fact 3.
Proof of Fact 3. We can achieve
in relation to its last row as
On the other hand, consider
to be the
i-th order principal sub-matrix of
, created by the last
i rows and columns,
. Let
. Then
,
and
We have adopted similar computation as described above.
Proof of Fact 4. We noticed that
is a total of all principal minors of
which have
rows and columns, then
where
In line with the above Equation (
10), we find
The following forms could be derived using the above equations.
and
Fact 3 is derived by substituting Equations (
13)–(
17) into (
12).
Lemma 3 follows directly from Equation and Facts 3 and 4. □
We can easily obtain the following theorems by combining Lemmas 4–6.
Theorem 1. Let be a generalized phenylenes with n hexagons and squares. Thenwhere Theorem 2. Let be a generalized phenylenes with n hexagons and squares. Then Proof. By Lemma 3, then we have
. Noted that
Hence, Theorem 2 immediately follows, along with Lemma 3. □