No Arabic abstract
Let $A in mathbb{R}^{n times n}$ be the adjacency matrix of an ErdH{o}s Renyi graph $G(n, d/n)$ for $d = omega(1)$ and $d leq 3log(n)$. We show that as $n$ goes to infinity, with probability that goes to $1$, the adjacency matrix of the $3$-core of $G(n, d/n)$ is invertible.
Graph shotgun assembly refers to the problem of reconstructing a graph from a collection of local neighborhoods. In this paper, we consider shotgun assembly of ErdH{o}s-Renyi random graphs $G(n, p_n)$, where $p_n = n^{-alpha}$ for $0 < alpha < 1$. We consider both reconstruction up to isomorphism as well as exact reconstruction (recovering the vertex labels as well as the structure). We show that given the collection of distance-$1$ neighborhoods, $G$ is exactly reconstructable for $0 < alpha < frac{1}{3}$, but not reconstructable for $frac{1}{2} < alpha < 1$. Given the collection of distance-$2$ neighborhoods, $G$ is exactly reconstructable for $0 < alpha < frac{1}{2}$, but not reconstructable for $frac{3}{4} < alpha < 1$.
Given an unlabeled graph $G$ on $n$ vertices, let ${N_{G}(v)}_{v}$ be the collection of subgraphs of $G$, where for each vertex $v$ of $G$, $N_{G}(v)$ is the subgraph of $G$ induced by vertices of $G$ of distance at most one from $v$. We show that there are universal constants $C,c>0$ with the following property. Let the sequence $(p_n)_{n=1}^infty$ satisfy $n^{-1/2}log^C nleq p_nleq c$. For each $n$, let $Gamma_n$ be an unlabeled $G(n,p_n)$ Erdos-Renyi graph. Then with probability $1-o(1)$, any unlabeled graph $tilde Gamma_n$ on $n$ vertices with ${N_{tilde Gamma_n}(v)}_{v}={N_{Gamma_n}(v)}_{v}$ must coincide with $Gamma_n$. This establishes $p_n= tilde Theta(n^{-1/2})$ as the transition for the density parameter $p_n$ between reconstructability and non-reconstructability of Erdos-Renyi graphs from their $1$--neighborhoods, answering a question of Gaudio and Mossel.
A (not necessarily proper) vertex colouring of a graph has clustering $c$ if every monochromatic component has at most $c$ vertices. We prove that planar graphs with maximum degree $Delta$ are 3-colourable with clustering $O(Delta^2)$. The previous best bound was $O(Delta^{37})$. This result for planar graphs generalises to graphs that can be drawn on a surface of bounded Euler genus with a bounded number of crossings per edge. We then prove that graphs with maximum degree $Delta$ that exclude a fixed minor are 3-colourable with clustering $O(Delta^5)$. The best previous bound for this result was exponential in $Delta$.
Let $G$ be an $n$-vertex graph and let $L:V(G)rightarrow P({1,2,3})$ be a list assignment over the vertices of $G$, where each vertex with list of size 3 and of degree at most 5 has at least three neighbors with lists of size 2. We can determine $L$-choosability of $G$ in $O(1.3196^{n_3+.5n_2})$ time, where $n_i$ is the number of vertices in $G$ with list of size $i$ for $iin {2,3}$. As a corollary, we conclude that the 3-colorability of any graph $G$ with minimum degree at least 6 can be determined in $O(1.3196^{n-.5Delta(G)})$ time.
We find precise asymptotic estimates for the number of planar maps and graphs with a condition on the minimum degree, and properties of random graphs from these classes. In particular we show that the size of the largest tree attached to the core of a random planar graph is of order c log(n) for an explicit constant c. These results provide new information on the structure of random planar graphs.