- Open Access
On the sum of the two largest Laplacian eigenvalues of trees
© Guan et al.; licensee Springer 2014
- Received: 23 December 2013
- Accepted: 5 June 2014
- Published: 23 June 2014
For , the sum of the two largest Laplacian eigenvalues of a tree T, an upper bound is obtained. Moreover, among all trees with vertices, the unique tree which attains the maximal value of is determined.
- upper bound
- Laplacian matrix
Let be the vertex set and be the edge set of a graph G. The numbers of vertices and edges of G are denoted by and , respectively. For a vertex , let be the set of vertices adjacent to v and be the degree of v. Particularly, denote by the maximum degree of G. The diameter of a connected graph G, denoted by , is the maximum distance among all pairs of vertices in G. Let be the adjacency matrix of G and be the diagonal matrix of vertex degrees. The matrix is called the Laplacian matrix of G and its eigenvalues are called the Laplacian eigenvalues of G. Let be the Laplacian eigenvalues of a graph G with n vertices. It is well known that and . In particular, is called the algebraic connectivity of G and it is denoted by .
is true for . Considering a tree T, we have . Recently, Fritscher et al. improved this bound by giving . This paper determines the extremal tree that attains the bound of . Moreover, for general connected graphs, we also give a conjecture on the extremal graphs for .
Let be the sum of the largest k Laplacian eigenvalues of a graph G. When , we shall write instead of for simplicity. For graphs G and H, we denote by the graph with vertex set and edge set . The following lemmas come from an important result as regards a real symmetric matrix.
Lemma 2.1 ()
Letbe some edge-disjoint graphs. Thenfor any k.
Lemma 2.2 ()
For any graph G, .
Lemma 2.4 ()
Let T be a tree of order n. If, then, with equality if and only if.
Corollary 2.5 Let T be a tree with n vertices and diameter. Then.
for . For , a straightforward calculation shows that . □
Lemma 2.6 ()
Let T be a tree of order n and diameter. Then, with equality if and only if.
Lemma 2.7 ()
Let G be a graph with a vertex u of degree one. Then.
Lemma 2.7 implies that the algebraic connectivity of a tree is not greater than that of its subtree.
Lemma 2.8 ()
Let () be a tree obtained from a starby replacing its k edges with k paths of length two, respectively. If, then.
The following lemma can be found in  and is known as the Interlacing Theorem of Laplacian eigenvalues.
Next we give the main theorem of this section. Its proof is divided into several sequent claims.
Theorem 2.10 For any tree T with order, . The equality holds if and only if.
Claim 2.11 For any tree T with orderand diameter, except that.
By virtue of Lemma 2.6, we have except that . Equivalently, except that .
Indeed, by (2) it suffices to show . Note that for , contains as a subtree. By Lemma 2.7, .
Also we have . □
Claim 2.12 For any tree T with order n and diameter, .
Proof Since , then and there is a path of length 5 in T. By inequality (3), it suffices to show . First suppose that there is a path in T such that either or . Let , be the two components of . Clearly, both and have at least two edges.
Claim 2.13 For any tree T with order n and diameter 4, .
Proof First suppose that T contains a path such that . Now and it suffices to show . Without loss of generality assume that . Let , be the two components of with . Then both and have at least two edges.
If , of attain at the same component, say , then similarly to inequalities (5) and (6), we can observe that .
This implies that .
When , is a path. Comparing with (4), . This completes the proof. □
Theorem 2.14 Let m, n be two positive integers withandbe a graph of order n and size m obtained from a given edge uv by joiningindependent vertices with u and v, respectively, and anotherindependent vertices with u. Then.
Proof Let be a graph obtained by joining a vertex to s vertices of a given complete graph of order and be its complement graph. Then is isomorphic to the union of and s isolated vertices. Clearly, the Laplacian eigenvalues of consist of , 1 with multiplicity and 0 with multiplicity . Recall that for any graph G with n vertices, for and . So the Laplacian eigenvalues of consist of with multiplicity s, with multiplicity , s and 0.
Now is isomorphic to the union of and an isolated vertex. So the Laplacian eigenvalues of consist of with multiplicity , with multiplicity , , and 0 with multiplicity 2. Therefore, the Laplacian eigenvalues of consist of n, , 2 with multiplicity , 1 with multiplicity and 0. So . □
Recall that for any graph G. When , Haemers’ bound is clearly not attainable. Theorem 2.14 implies that if , Haemers’ bound is always sharp for connected graphs other than trees. Ending the paper, we present a conjecture on the uniqueness of the extremal graph.
Conjecture 2.15 Among all connected graphs with n vertices andedges, is the unique graph with maximal value of.
The authors are grateful to the referees for carefully reading the manuscript and for providing some comments and suggestions, which led to improvements in the paper. The research was supported by the National Natural Science Foundation of China (11101057, 11201432), the Foundation for Young Talents in College of Anhui Province (2012SQRL170) and the Natural Science Foundation of Anhui Province (1308085MA03).
- Bıyıkoǧlu T, Leydold J: Algebraic connectivity and degree sequences of trees. Linear Algebra Appl. 2009, 430: 811–817. 10.1016/j.laa.2008.09.030MathSciNetView ArticleGoogle Scholar
- Fallat S, Kirkland S: Extremizing algebraic connectivity subject to graph theoretic constraints. Electron. J. Linear Algebra 1998, 3: 48–74.MathSciNetView ArticleGoogle Scholar
- Fan YZ, Xu J, Wang Y, Liang D: The Laplacian spread of a tree. Discrete Math. Theor. Comput. Sci. 2008,10(1):79–86.MathSciNetGoogle Scholar
- Guo JM: On the second largest Laplacian eigenvalue of trees. Linear Algebra Appl. 2005, 404: 251–261.MathSciNetView ArticleGoogle Scholar
- Shao JY, Zhang L, Yuan XY: On the second Laplacian eigenvalues of trees of odd order. Linear Algebra Appl. 2006, 419: 475–485. 10.1016/j.laa.2006.05.021MathSciNetView ArticleGoogle Scholar
- Zhang XD, Li JS: The two largest eigenvalues of Laplacian matrices of trees. J. Univ. Sci. Technol. China 1998, 28: 513–518.Google Scholar
- Anderson WN, Morley TD: Eigenvalues of the Laplacian of a graph. Linear Multilinear Algebra 1985, 18: 141–145. 10.1080/03081088508817681MathSciNetView ArticleGoogle Scholar
- Haemers WH, Mohammadian A, Tayfeh-Rezaie B: On the sum of Laplacian eigenvalues of graphs. Linear Algebra Appl. 2010, 432: 2214–2221. 10.1016/j.laa.2009.03.038MathSciNetView ArticleGoogle Scholar
- Brouwer AE, Haemers WH: Spectra of Graphs. Springer, New York; 2012.View ArticleGoogle Scholar
- Fritscher E, Hoppen C, Rocha I, Trevisan V: On the sum of the Laplacian eigenvalues of a tree. Linear Algebra Appl. 2011, 435: 371–399. 10.1016/j.laa.2011.01.036MathSciNetView ArticleGoogle Scholar
- Grone R, Merris R, Sunder VS: The Laplacian spectrum of a graph II. SIAM J. Matrix Anal. Appl. 1990, 11: 218–238. 10.1137/0611016MathSciNetView ArticleGoogle Scholar
- Merris R: A note on the Laplacian graph eigenvalues. Linear Algebra Appl. 1998, 285: 33–35. 10.1016/S0024-3795(98)10148-9MathSciNetView ArticleGoogle Scholar
- Godsil C, Royle G: Algebraic Graph Theory. Springer, New York; 2001.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.