1. Introduction
Let
G be a connected graph with
and edge set
, where
. For
,
denotes the distance between vertices
and
. In particular,
for any
. Let
be the distance matrix of graph
G. Let
denote the eigenvalues of
with non-increasing order. Also let
be the distance spectral radius of a graph
G. When more than one graph is under consideration, then we write
instead of
. By the Perron-Frobenius theorem, a unique unit eigenvector
corresponding to the largest distance eigenvalue of
G has all positive eigencomponents, which is called the Perron vector of
. Several studies on this topic have been conducted, see [
1,
2,
3,
4,
5,
6,
7] and the survey [
8].
The energy of a graph
G, often denoted by
, is defined to be the sum of the absolute value of the eigenvalues of its adjacency matrix of a graph. The energy of a graph was first defined by Ivan Gutman in 1978 [
9]. However, the motivation for his definition appeared much earlier, in the 1930’s, when Erich Hückel proposed the famous Hückel Molecular Orbital Theory. Hückel’s method allows chemists to approximate energies associated with
-electron orbitals in a special class of molecules called conjugated hydrocarbons. From the motivation of graph energy, Indulal et al. [
10] proposed the distance energy
of
G which is defined by
Since the trace of
is zero, we have
Extremal graph theory is one of the important topics in graph theory and combinatorics. In extremal graph theory to find the extremal (maximal and/or minimal) graphs for some graph invariant is the important and interesting problem. It is very difficult to find the exact value of the distance energy of graphs. In this paper we are concentrating to obtain the maximal distance energy for some special class of graphs. For the basic mathematical properties of
, including various lower and upper bounds, see [
8,
11,
12,
13,
14] and the references therein.
If the maximal connected subgraph of G has no cut vertex then it is called a block. If each block of graph G is a clique then G is called clique tree. Let be a path graph of order . A clique path, denoted by , is a graph which is obtained from by replacing each edge of by a clique () such that for and . If , we use to denote for short.
Lin et al. [
15] discussed several properties of clique trees and discovered that the positive inertia and the negative inertia of the distance matrix of a clique tree with
n vertices are 1 and
, respectively. Among all clique trees with order
n, the graph with the minimum distance energy has been characterized in [
15]. They also gave a conjecture related to the maximum distance energy as follows:
Conjecture 1 ([
15])
. The graph gives the maximum distance energy among all clique trees with cliques and order n.
Due to the fact that any clique tree has exactly one positive distance eigenvalue, then Conjecture 1 is equivalent to the following:
Conjecture 2. The graph gives the maximum largest distance eigenvalue among all clique trees with cliques and order n.
Please note that Conjecture 2 has been proved for
[
4]. In the next section, we will confirm that Conjecture 2 holds for the remaining cases and thus Conjecture 1 follows immediately.
2. Proof of Conjecture 2
In [
15], the authors revealed that the clique trees with the maximum spectral radius belong to a special class of clique paths.
Lemma 1 ([
15])
. The graph with respect to the restriction gives the maximum largest distance eigenvalue among all clique trees with cliques and order n. By Lemma 1, for proving Conjecture 2, we only need to find the maximum distance spectral radius of
, where
. Please note that Conjecture 2 follows directly if
. For
, we have
and hence
gives the maximum distance spectral radius among all clique trees with cliques
and order
n, which provide the Conjecture 2 is true. Therefore, hereafter we only consider the cases with
. Now let
with
, whose vertex set is
and edge set is
, where
,
and
. Let
be the unit positive eigenvector corresponding to spectral radius
of
. By symmetry, one can see that
and
. Thus, we may suppose that
From
First, we discuss the properties of Perron vector of .
Lemma 2. Let with (p is an integer, ) and . Letbe the Perron vector of . Then for . Moreover, if , then Proof. Since
, from (
1), we obtain
and if
,
where
. Setting
in the above, we get
and if
,
where
. Combining (
2) and setting
in (
4) with
, we have
that is,
as
.
Using (
3), from (
4),
that is,
From (
6), we conclude that if
is positive (zero or negative) then all
are positive (zero or negative, respectively). □
Proof of Claim 1. We suppose that
. This implies that
, thus
Using
in (
5), we obtain that
It implies that
as
,
and
. Combining the last inequality with (
3), we conclude that
, which is a contradiction. We finish the proof of Claim 1.
Using Claim 1, we conclude that
, i.e.,
for
. From (
6), we obtain
, i.e.,
for
. □
By the same method as the above, we also obtain the similar result for odd k.
Lemma 3. Let with (p is an integer, ) and . Letdenote the Perron vector of . Then for . Combining Lemmas 2 and 3, the next theorem follows immediately.
Theorem 1. Let with and . Letbe Perron vector of . If , then ; otherwise, for . Theorem 2. For any pair of integers s and t with , Proof. For convenience, let
and let
. Let
be the Perron vector corresponding to spectral radius
of
. Then
Now let
, where
To prove
, we have to prove that
. From
,
and
Combining the above two equations with
, we get
that is,
By Theorem 1 with (
8),
. We can rewrite (
7) as
Using Theorem 1 with
and (
8), from the above equation, we obtain
that is,
where
□
Claim 2. .
Proof of Claim 2. Let
Recall that
with
. For
,
. For
,
. For
,
as
and
Please note that
. Combining the above results, we obtain
It is well-known that . Thus, we get the result in Claim 2.
By combining this result with (
9),
. Hence we get the required result. □
Remark 1. By Theorem 2, we immediately confirm that Conjecture 2 is true. Hence, the Conjecture 1 is confirmed to be true as well.