Critical percolation on Cayley graphs of groups acting on trees

This article presents a method for finding the critical probability pc for the Bernoulli bond percolation on graphs with the so called tree-like structure. Such graphs can be decomposed into a tree of pieces which have finitely many isomorphism classes. This class of graphs includes the Cayley graphs of amalgamated products, HNN extensions or general groups acting on trees. It also includes all transitive graphs with more than one end. The idea of the method is to find a multi-type Galton-Watson branching process (with a parameter p) which has finite expected population size if and only if the expected percolation cluster size is finite. This provides a sufficient information about pc. In particular if the pairwise intersections of pieces are finite, then pc is the smallest positive p for which det(M − 1) = 0, where M is the first-moment matrix of the branching process. If the pieces of the tree-like structure are finite, then pc is an algebraic number, and we give an algorithm computing pc as a root of some algebraic function. We show that any Cayley graph of a group acting on a tree with finite vertex stabilizers with respect to any finite generating set has a tree-like structure with finite pieces. In particular we show how to compute pc of the Cayley graph of a free group with respect to any finite generating set.