The evolution of random graphs on surfaces

Abstract

For integers $g,m \geq 0$ and $n>0$, let $S_{g}(n,m)$ denote the graph taken uniformly at random from the set of all graphs on $\{1,2, \ldots, n\}$ with exactly $m=m(n)$ edges and with genus at most $g$. We use counting arguments to investigate the components, subgraphs, maximum degree, and largest face size of $S_{g}(n,m)$, finding that there is often different asymptotic behaviour depending on the ratio $\frac{m}{n}$. In our main results, we show that the probability that $S_{g}(n,m)$ contains any given non-planar component converges to $0$ as $n \to \infty$ for all $m(n)$; the probability that $S_{g}(n,m)$ contains a copy of any given planar graph converges to $1$ as $n \to \infty$ if $\liminf \frac{m}{n} > 1$; the maximum degree of $S_{g}(n,m)$ is $\Theta (\ln n)$ with high probability if $\liminf \frac{m}{n} > 1$; and the largest face size of $S_{g}(n,m)$ has a threshold around $\frac{m}{n}=1$ where it changes from $\Theta (n)$ to $\Theta (\ln n)$ with high probability.