Hopf decomposition and horospheric limit sets
Vadim A. Kaimanovich

By looking at the relationship between the recurrence properties of a countable group action with a quasi-invariant measure and the structure of its ergodic components we establish a simple general description of the Hopf decompo- sition of the action into the conservative and the dissipative parts in terms of the Radon-Nikodym derivatives of the action.

As an application we prove that the conservative part of the boundary action of a discrete group of isometries of a Gromov hyperbolic space with respect to any invariant quasi-conformal stream coincides (mod 0) with the big horospheric limit set of the group. As an example we consider in more detail the Hopf decompo- sition in the simplest model case: a subgroup of a finitely generated free group acting on the boundary of the ambient group.