The phase transition in site percolation on pseudo-random graphs

We establish the existence of the phase transition in site percolation on pseudo-random $d$-regular graphs. Let $G=(V,E)$ be an $(n,d,λ)$-graph, that is, a $d$-regular graph on $n$ vertices in which all eigenvalues of the adjacency matrix, but the first one, are at most $λ$ in their absolute values. Form a random subset $R$ of $V$ by putting every vertex $v\in V$ into $R$ independently with probability $p$. Then for any small enough constant $ε>0$, if $p=\frac{1-ε}{d}$, then with high probability all connected components of the subgraph of $G$ induced by $R$ are of size at most logarithmic in $n$, while for $p=\frac{1+ε}{d}$, if the eigenvalue ratio $λ/d$ is small enough as a function of $ε$, then typically $R$ spans a connected component of size at least $\frac{εn}{d}$ and a path of length proportional to $\frac{ε^2n}{d}$.

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