Laplacian Distribution and Domination

Let $m_G(I)$ denote the number of Laplacian eigenvalues of a graph $G$ in an interval $I$, and let $\gamma(G)$ denote its domination number. We extend the recent result $m_G[0,1) \leq \gamma(G)$, and show that isolate-free graphs also satisfy $\gamma(G) \leq m_G[2,n]$. In pursuit of better understanding Laplacian eigenvalue distribution, we find applications for these inequalities. We relate these spectral parameters with the approximability of $\gamma(G)$, showing that $\frac{\gamma(G)}{m_G[0,1)} \not\in O(\log n)$. However, $\gamma(G) \leq m_G[2, n] \leq (c + 1) \gamma(G)$ for $c$-cyclic graphs, $c \geq 1$. For trees $T$, $\gamma(T) \leq m_T[2, n] \leq 2 \gamma(G)$

On incidence energy of a graph

AbstractThe Laplacian-energy like invariant LEL(G) and the incidence energy IE(G) of a graph are recently proposed quantities, equal, respectively, to the sum of the square roots of the Laplacian eigenvalues, and the sum of the singular values of the incidence matrix of the graph G. However, IE(G) is closely related with the eigenvalues of the Laplacian and signless Laplacian matrices of G. For bipartite graphs, IE=LEL. We now point out some further relations for IE and LEL: IE can be expressed in terms of eigenvalues of the line graph, whereas LEL in terms of singular values of the incidence matrix of a directed graph. Several lower and upper bounds for IE are obtained, including those that pertain to the line graph of G. In addition, Nordhaus–Gaddum-type results for IE are established

Bounds of the Spectral Radius and the Nordhaus-Gaddum Type of the Graphs

The Laplacian spectra are the eigenvalues of Laplacian matrix L(G)=D(G)-A(G), where D(G) and A(G) are the diagonal matrix of vertex degrees and the adjacency matrix of a graph G, respectively, and the spectral radius of a graph G is the largest eigenvalue of A(G). The spectra of the graph and corresponding eigenvalues are closely linked to the molecular stability and related chemical properties. In quantum chemistry, spectral radius of a graph is the maximum energy level of molecules. Therefore, good upper bounds for the spectral radius are conducive to evaluate the energy of molecules. In this paper, we first give several sharp upper bounds on the adjacency spectral radius in terms of some invariants of graphs, such as the vertex degree, the average 2-degree, and the number of the triangles. Then, we give some numerical examples which indicate that the results are better than the mentioned upper bounds in some sense. Finally, an upper bound of the Nordhaus-Gaddum type is obtained for the sum of Laplacian spectral radius of a connected graph and its complement. Moreover, some examples are applied to illustrate that our result is valuable

Beyond graph energy: norms of graphs and matrices

In 1978 Gutman introduced the energy of a graph as the sum of the absolute values of graph eigenvalues, and ever since then graph energy has been intensively studied. Since graph energy is the trace norm of the adjacency matrix, matrix norms provide a natural background for its study. Thus, this paper surveys research on matrix norms that aims to expand and advance the study of graph energy. The focus is exclusively on the Ky Fan and the Schatten norms, both generalizing and enriching the trace norm. As it turns out, the study of extremal properties of these norms leads to numerous analytic problems with deep roots in combinatorics. The survey brings to the fore the exceptional role of Hadamard matrices, conference matrices, and conference graphs in matrix norms. In addition, a vast new matrix class is studied, a relaxation of symmetric Hadamard matrices. The survey presents solutions to just a fraction of a larger body of similar problems bonding analysis to combinatorics. Thus, open problems and questions are raised to outline topics for further investigation.Comment: 54 pages. V2 fixes many typos, and gives some new materia

Cotas para a soma de autovalores de grafos

Neste trabalho, investigamos problemas envolvendo desigualdades para os autovalores das matrizes Laplaciana e Laplaciana sem sinal. Estudamos o problema de Nordhaus-Gaddum e obtemos resultados para os dois maiores autovalores da matriz Laplaciana e para o segundo maior e menor autovalores da matriz Laplaciana sem sinal. Na maioria dos casos, garantimos que as desigualdades obtidas são os melhores possíveis. Apresentamos uma técnica para obter uma cota superior para a soma dos k maiores autovalores da matriz Laplaciana sem sinal de classes de grafos que possuam uma cota superior específica para o maior autovalor dessa matriz. Em 2013, F. Ashraf et al. [7] propuseram uma versão da conjectura de Brouwer para a matriz Laplaciana sem sinal. Essa conjectura foi provada para diversos casos, mas não possui uma demonstração para o caso geral. Investigamos sua validade para os cografos e grafos threshold, apresentando alguns resultados parciais.In this work, we investigate problems involving inequalities for the eigenvalues of the Laplacian and signless Laplacian matrices. We studied the Nordhaus- Gaddum problem and obtained results for the two largest eigenvalues of the Laplacian matrix and for the second largest and smallest eigenvalues of the signless Laplacian matrix. In most cases, we guarantee that the inequalities obtained are best possible. We present a technique to obtain an upper bound for the sum of the k largest eigenvalues of the signless Laplacian matrix of classes of graphs that have a specific upper bound for the largest eigenvalue of that matrix. In 2013, F. Ashraf et al. [7] proposed a version of Brouwer conjecture for the signless Laplacian matrix. This conjecture has been proved for several cases, but it does not have a proof for the general case. We investigated its validity for cographs and threshold graphs, presenting some partial results

The number of spanning trees of a graph

Let G be a simple connected graph of order n, m edges, maximum degree Delta(1) and minimum degree delta. Li et al. (Appl. Math. Lett. 23: 286-290, 2010) gave an upper bound on number of spanning trees of a graph in terms of n, m, Delta(1) and delta: t(G) <= delta (2m-Delta(1)-delta-1/n-3)(n-3). The equality holds if and only if G congruent to K-1,K-n-1, G congruent to K-n, G congruent to K-1 boolean OR (K-1 boolean OR Kn-2) or G congruent to K-n - e, where e is any edge of K-n. Unfortunately, this upper bound is erroneous. In particular, we show that this upper bound is not true for complete graph K-n. In this paper we obtain some upper bounds on the number of spanning trees of graph G in terms of its structural parameters such as the number of vertices (n), the number of edges (m), maximum degree (Delta(1)), second maximum degree (Delta(2)), minimum degree (delta), independence number (alpha), clique number (omega). Moreover, we give the Nordhaus-Gaddum-type result for number of spanning trees.Faculty research Fund, Sungkyunkwan UniversityKorean Government (2013R1A1A2009341)Selçuk ÜniversitesiGlaucoma Research FoundationHong Kong Baptist Universit

New upper bounds for Laplacian energy

Abstract We obtain upper bounds and Nordhaus-Gaddum-type results for the Laplacian energy. The bounds in terms of the number of vertices are asymptotically best possible

Proximity and Remoteness in Graphs: a survey

The proximity $\pi = \pi (G)$ of a connected graph $G$ is the minimum, over all vertices, of the average distance from a vertex to all others. Similarly, the maximum is called the remoteness and denoted by $\rho = \rho (G)$. The concepts of proximity and remoteness, first defined in 2006, attracted the attention of several researchers in Graph Theory. Their investigation led to a considerable number of publications. In this paper, we present a survey of the research work.Comment: arXiv admin note: substantial text overlap with arXiv:1204.1184 by other author