Cycle lengths in sparse random graphs

Abstract: We study the set ${\cal L}(G)$ of lengths of all cycles that appear in a random $d$-regular $G$ on $n$ vertices for a fixed $d\geq 3$, as well as in Erd\H{o}s--R\'enyi random graphs on $n$ vertices with a fixed average degree $c>1$. Fundamental results on the distribution of cycle counts in these models were established in the 1980's and early 1990's, with a focus on the extreme lengths: cycles of fixed length, and cycles of length linear in $n$. Here we derive, for a random $d$-regular graph, the limiting probability that ${\cal L}(G)$ simultaneously contains the entire range ${\ell,\ldots,n}$ for $\ell\geq 3$, as an explicit expression $\theta_\ell=\theta_\ell(d)\in(0,1)$ which goes to $1$ as $\ell\to\infty$. For the random graph ${\cal G}(n,p)$ with $p=c/n$, where $c\geq C_0$ for some absolute constant $C_0$, we show the analogous result for the range ${\ell,\ldots,(1-o(1))L_{\max}(G)}$, where $L_{\max}$ is the length of a longest cycle in $G$. The limiting probability for ${\cal G}(n,p)$ coincides with $\theta_\ell$ from the $d$-regular case when $c$ is the integer $d-1$. In addition, for the directed random graph ${\cal D}(n,p)$ we show results analogous to those on ${\cal G}(n,p)$, and for both models we find an interval of $c \epsilon^2 n$ consecutive cycle lengths in the slightly supercritical regime $p=\frac{1+\epsilon}n$.