Percolation in an ultrametric space

We study percolation on the hierarchical lattice of order $N$ where the probability of connection between two points separated by distance $k$ is of the form $c_k/N^{k(1+\delta)},\; \delta >-1$. Since the distance is an ultrametric, there are significant differences with percolation on the Euclidean lattice. There are two non-critical regimes: $\delta <1$, where percolation occurs, and $\delta >1$, where it does not occur. In the critical case, $\delta =1$, we use an approach in the spirit of the renormalization group method of statistical physics and connectivity results of Erd\H{o}s-Renyi random graphs play a key role. We find sufficient conditions on $c_k$ such that percolation occurs, or that it does not occur. An intermediate situation called pre-percolation is also considered. In the cases of percolation we prove uniqueness of the constructed percolation clusters. In a previous paper \cite{DG1} we studied percolation in the $N\to\infty$ limit (mean field percolation) which provided a simplification that allowed finding a necessary and sufficient condition for percolation. For fixed $N$ there are open questions, in particular regarding the existence of a critical value of a parameter in the definition of $c_k$, and if it exists, what would be the behaviour at the critical point.