Small rainbow cliques in randomly perturbed dense graphs

For two graphs $G$ and $H$, write $G stackrel{mathrm{rbw}}{longrightarrow} H$ if $G$ has the property that every emph{proper} colouring of its edges yields a emph{rainbow} copy of $H$. We study the thresholds for such so-called emph{anti-Ramsey} properties in randomly perturbed dense graphs, which are unions of the form $G cup mathbb{G}(n,p)$, where $G$ is an $n$-vertex graph with edge-density at least $d >0$, and $d$ is independent of $n$. In a companion article, we proved that the threshold for the property $G cup mathbb{G}(n,p) stackrel{mathrm{rbw}}{longrightarrow} K_ell$ is $n^{-1/m_2(K_{leftlceil ell/2 rightrceil})}$, whenever $ell geq 9$. For smaller $ell$, the thresholds behave more erratically, and for $4 le ell le 7$ they deviate downwards significantly from the aforementioned aesthetic form capturing the thresholds for emph{large} cliques. In particular, we show that the thresholds for $ell in {4, 5, 7}$ are $n^{-5/4}$, $n^{-1}$, and $n^{-7/15}$, respectively. For $ell in {6, 8}$ we determine the threshold up to a $(1 + o(1))$-factor in the exponent: they are $n^{-(2/3 + o(1))}$ and $n^{-(2/5 + o(1))}$, respectively. For $ell = 3$, the threshold is $n^{-2}$; this follows from a more general result about odd cycles in our companion paper.