Rainbow spanning trees in random edge-colored graphs

A well known result of Erd\H{o}s and R\'enyi states that if $p = \frac{c \log n}{n}$ and $G$ is a random graph constructed from $G(n,p)$, $G$ is a.a.s. disconnected when $c 1$. When $c > 1$, we may equivalently say that $G$ a.a.s. contains a spanning tree. We find analogous thresholds in the setting of random edge-colored graphs. Specifically, we consider a family $\mathcal G$ of $n-1$ graphs on a common set $X$ of $n$ vertices, each of a different color, and each randomly chosen from $G(n,p)$, with $p = \frac{c \log n}{n^2}$. We show that when $c > 2$, there a.a.s. exists a spanning tree on $X$ using exactly one edge of each color, and we show that such a spanning tree a.a.s. does not exist when $c < 2$.Comment: It was discovered that the main result follows from Frieze, McKay, "Multicolored trees in random graphs," RSA, 199

Applications of Geometric and Spectral Methods in Graph Theory

Networks, or graphs, are useful for studying many things in today’s world. Graphs can be used to represent connections on social media, transportation networks, or even the internet. Because of this, it’s helpful to study graphs and learn what we can say about the structure of a given graph or what properties it might have. This dissertation focuses on the use of the probabilistic method and spectral graph theory to understand the geometric structure of graphs and find structures in graphs. We will also discuss graph curvature and how curvature lower bounds can be used to give us information about properties of graphs. A rainbow spanning tree in an edge-colored graph is a spanning tree in which each edge is a different color. Carraher, Hartke, and Horn showed that for n and C large enough, if G is an edge-colored copy of Kn in which each color class has size at most n/2, then G has at least [n/(C log n)] edge-disjoint rainbow spanning trees. Here we show that spectral graph theory can be used to prove that if G is any edge-colored graph with n vertices in which each color appears on at most δλ1/2 edges, where δ ≥ C log n for n and C sufficiently large and λ1 is the second-smallest eigenvalue of the normalized Laplacian matrix of G, then G contains at least [δλ1/ C log n] edge-disjoint rainbow spanning trees. We show how curvature lower bounds can be used in the context of understanding (personalized) PageRank, which was developed by Brin and Page. PageRank ranks the importance of webpages near a seed webpage, and we are interested in how this importance diffuses. We do this by using a notion of graph curvature introduced by Bauer, Horn, Lin, Lippner, Mangoubi, and Yau

Degree Sequences of Edge-Colored Graphs in Specified Families and Related Problems

Movement has been made in recent times to generalize the study of degree sequences to k-edge-colored graphs and doing so requires the notion of a degree vector\u3c\italic\u3e. The degree vector of a vertex v\u3c\italic\u3e in a k-edge-colored graph is a column vector in which entry i\u3c\italic\u3e indicates the number of edges of color i\u3c\italic\u3e incident to v\u3c\italic\u3e. Consider the following question which we refer to as the \u3c\italic\u3ek-Edge-Coloring Problem\u3c\italic\u3e. Given a set of column vectors C\u3c\italic\u3e and a graph family F\u3c\italic\u3e, when does there exist some k-edge-colored graph in F\u3c\italic\u3e whose set of degree vectors is C\u3c\italic\u3e? This question is NP-Complete in general but certain graph families yield tractable results. In this document, I present results on the k-Edge-Coloring Problem and the related Factor Problem for the following families of interest: unicyclic graphs, disjoint unions of paths (DUPs), disjoint union of cycles (DUCs), grids, and 2-trees

Color-avoiding percolation in edge-colored Erd\H{o}s-R\'enyi graphs

We study a variant of the color-avoiding percolation model introduced by Krause et al., namely we investigate the color-avoiding bond percolation setup on (not necessarily properly) edge-colored Erd\H{o}s-R\'{e}nyi random graphs. We say that two vertices are color-avoiding connected in an edge-colored graph if after the removal of the edges of any color, they are in the same component in the remaining graph. The color-avoiding connected components of an edge-colored graph are maximal sets of vertices such that any two of them are color-avoiding connected. We consider the fraction of vertices contained in color-avoiding connected components of a given size as well as the fraction of vertices contained in the giant color-avoiding connected component. Under some mild assumptions on the color-densities, we prove that these quantities converge and the limits can be expressed in terms of probabilities associated to edge-colored branching process trees. We provide explicit formulas for the limit of the normalized size of the giant color-avoiding component, and in the two-colored case we also provide explicit formulas for the limit of the fraction of vertices contained in color-avoiding connected components of a given size.Comment: 59 pages + Appendix + List of notation. Added reference to the recent arXiv preprint arXiv:2211.1608

Spanning trees with many or few colors in edge-colored graphs

Given a graph G = (V,E) and a (not necessarily proper) edge-coloring of G, we consider the complexity of finding a spanning tree of G with as many different colors as possible, and of finding one with as few different colors as possible. We show that the first problem is equivalent to finding a common independent set of maximum cardinality in two matroids, implying that there is a polynomial algorithm. We use the minimum dominating set problem to show that the second problem is NP-hard