Eigenvalue bounds for the signless laplacian

We extend our previous survey of properties of spectra of signless Laplacians of graphs. Some new bounds for eigenvalues are given, and the main result concerns the graphs whose largest eigenvalue is maximal among the graphs with fixed numbers of vertices and edges. The results are presented in the context of a number of computer-generated conjectures

Exploration of graphs with excluded minors

We study the online graph exploration problem proposed by Kalyanasundaram and Pruhs (1994) and prove a constant competitive ratio on minor-free graphs. This result encompasses and significantly extends the graph classes that were previously known to admit a constant competitive ratio. The main ingredient of our proof is that we find a connection between the performance of the particular exploration algorithm Blocking and the existence of light spanners. Conversely, we exploit this connection to construct light spanners of bounded genus graphs. In particular, we achieve a lightness that improves on the best known upper bound for genus g>0 and recovers the known tight bound for the planar case (g=0).Comment: to appear at ESA 202

Exploration of Graphs with Excluded Minors

We study the online graph exploration problem proposed by Kalyanasundaram and Pruhs (1994) and prove a constant competitive ratio on minor-free graphs. This result encompasses and significantly extends the graph classes that were previously known to admit a constant competitive ratio. The main ingredient of our proof is that we find a connection between the performance of the particular exploration algorithm Blocking and the existence of light spanners. Conversely, we exploit this connection to construct light spanners of bounded genus graphs. In particular, we achieve a lightness that improves on the best known upper bound for genus g ? 1 and recovers the known tight bound for the planar case (g = 0)

An updated survey on rainbow connections of graphs - a dynamic survey

The concept of rainbow connection was introduced by Chartrand, Johns, McKeon and Zhang in 2008. Nowadays it has become a new and active subject in graph theory. There is a book on this topic by Li and Sun in 2012, and a survey paper by Li, Shi and Sun in 2013. More and more researchers are working in this field, and many new papers have been published in journals. In this survey we attempt to bring together most of the new results and papers that deal with this topic. We begin with an introduction, and then try to organize the work into the following categories, rainbow connection coloring of edge-version, rainbow connection coloring of vertex-version, rainbow $k$-connectivity, rainbow index, rainbow connection coloring of total-version, rainbow connection on digraphs, rainbow connection on hypergraphs. This survey also contains some conjectures, open problems and questions for further study

Sampling Random Colorings of Sparse Random Graphs

We study the mixing properties of the single-site Markov chain known as the Glauber dynamics for sampling $k$-colorings of a sparse random graph $G(n,d/n)$ for constant $d$. The best known rapid mixing results for general graphs are in terms of the maximum degree $\Delta$ of the input graph $G$ and hold when $k>11\Delta/6$ for all $G$. Improved results hold when $k>\alpha\Delta$ for graphs with girth $\geq 5$ and $\Delta$ sufficiently large where $\alpha\approx 1.7632\ldots$ is the root of $\alpha=\exp(1/\alpha)$; further improvements on the constant $\alpha$ hold with stronger girth and maximum degree assumptions. For sparse random graphs the maximum degree is a function of $n$ and the goal is to obtain results in terms of the expected degree $d$. The following rapid mixing results for $G(n,d/n)$ hold with high probability over the choice of the random graph for sufficiently large constant~$d$. Mossel and Sly (2009) proved rapid mixing for constant $k$, and Efthymiou (2014) improved this to $k$ linear in~$d$. The condition was improved to $k>3d$ by Yin and Zhang (2016) using non-MCMC methods. Here we prove rapid mixing when $k>\alpha d$ where $\alpha\approx 1.7632\ldots$ is the same constant as above. Moreover we obtain $O(n^{3})$ mixing time of the Glauber dynamics, while in previous rapid mixing results the exponent was an increasing function in $d$. As in previous results for random graphs our proof analyzes an appropriately defined block dynamics to "hide" high-degree vertices. One new aspect in our improved approach is utilizing so-called local uniformity properties for the analysis of block dynamics. To analyze the "burn-in" phase we prove a concentration inequality for the number of disagreements propagating in large blocks

On the local metric dimension of graphs

La dimensió mètrica d'un espai mètric general es va introduir el 1953, però va atreure poca atenció fins que, uns vint anys més tard, es va aplicar a les distàncies entre els vèrtexs d'un graf. Des de llavors s'ha utilitzat amb freqüència en la teoria de grafs, la química, la biologia, la robòtica i moltes altres disciplines. A causa de la multiplicitat de situacions de les que pot sorgir el problema de distingir els vèrtexs d'un graf, diverses variants del concepte original de la dimensió mètrica ha anat apareixent en la literatura especialitzada. En aquesta tesi s'estudia una d'aquestes variants, és a dir, la dimensió mètrica local. En concret, aquesta tesi ens centrem en el problema de calcular la dimensió mètrica local de grafs. En primer lloc, presentem l'estat de l'art de la dimensió mètrica local i obtenim alguns resultats originals en els que caracteritzem tots els grafs que atenyen algunes fites conegudes. En segon lloc, obtenim fórmules tancades i fites tenses per a la dimensió mètrica local de diverses famílies de grafs, incloent grafs producte fort, grafs corona i grafs producte lexicogràfic. Finalment, introduïm l'estudi de la dimensió mètrica local simultània i donem alguns resultats generals en aquesta nova línia d'investigació.La dimensión métrica de un espacio métrico general fue introducida en 1953, pero atrajo poca atención hasta que, aproximadamente veinte años más tarde, se aplicó a las distancias entre vértices de un gráfico. Desde entonces se ha utilizado con frecuencia en la teoría de los gráficos, la química, la biología, la robótica y muchas otras disciplinas. Debido a la multiplicidad de situaciones desde las que puede surgir el problema de distinguir los vértices de un gráfico, en la literatura especializada han aparecido varias variantes del concepto original de dimensión métrica. En esta tesis estudiamos una de estas variantes, a saber, la dimensión métrica local. En particular, nos centramos en el problema de calcular la dimensión métrica local de grafos. Primero presentamos el estado del arte de la dimensión métrica local y presentamos algunos resultados originales en los que caracterizamos todos los grafos que alcanzan algunas de las cotas conocidas. En segundo lugar, obtenemos fórmulas cerradas y cotas tensas para la dimensión métrica local de varias familias de grafos, entre los que destacamos los grafos producto fuerte, grafos corona y grafos producto lexicográfico. Finalmente, introducimos el estudio de la dimensión métrica local simultánea y damos algunos resultados generales en esta nueva línea de investigación.The metric dimension of a general metric space was introduced in 1953 but attracted little attention until, about twenty years later, it was applied to the distances between vertices of a graph. Since then it has been frequently used in graph theory, chemistry, biology, robotics and many other disciplines. Due to the variety of situations from which the problem of distinguishing the vertices of a graph can arise, several variants of the original concept of metric dimension have been appearing in specialized literature. In this thesis we study one of these variants, namely, the local metric dimension. Specifically, we focus on the problem of computing the local metric dimension of graphs. We first report on the state of the art on the local metric dimension and present some original results in which we characterize all graphs that reach some known bounds. Secondly, we obtain closed formulas and tight bounds on the local metric dimension of several families of graphs, including strong product graphs, corona product graphs, rooted product graphs and lexicographic product graphs. Finally, we introduce the study of simultaneous local metric dimension and we give some general results on this new research line