We introduce and study the space
${{\mathcal{S}{\rm Curr} (F_N)}}$ of
subset currents on the free group
F N , and, more generally, on a word-hyperbolic group. A subset current on
F N is a positive
F N -invariant locally finite Borel measure on the space
${{\mathfrak{C}_N}}$ of all closed subsets of
?F N consisting of at least two points. The well-studied space Curr(
F N ) of geodesics currents–positive
F N -invariant locally finite Borel measures defined on pairs of different boundary points–is contained in the space of subset currents as a closed
${{\mathbb{R}}}$ -linear Out(
F N )-invariant subspace. Much of the theory of Curr(
F N ) naturally extends to the
${{\mathcal{S}\;{\rm Curr} (F_N)}}$ context, but new dynamical, geometric and algebraic features also arise there. While geodesic currents generalize conjugacy classes of nontrivial group elements, a subset current is a measure-theoretic generalization of the conjugacy class of a nontrivial finitely generated subgroup in
F N . If a free basis
A is fixed in
F N , subset currents may be viewed as
F N -invariant measures on a “branching” analog of the geodesic flow space for
F N , whose elements are infinite subtrees (rather than just geodesic lines) of the Cayley graph of
F N with respect to
A. Similarly to the case of geodesics currents, there is a continuous Out(
F N )-invariant “co-volume form” between the Outer space cv
N and the space
${{\mathcal{S}\;{\rm Curr} (F_N)}}$ of subset currents. Given a tree
${{T \in {\rm cv}_N}}$ and the “counting current”
${{\eta_H \in \mathcal{S}\;{\rm Curr} (F_N)}}$ corresponding to a finitely generated nontrivial subgroup
H ≤
F N , the value
${{\langle T, \eta_H \rangle}}$ of this intersection form turns out to be equal to the co-volume of
H, that is the volume of the metric graph
T H /
H, where
${{T_H \subseteq T}}$ is the unique minimal
H-invariant subtree of
T. However, unlike in the case of geodesic currents, the co-volume form
${{{\rm cv}_N \times \mathcal{S}\;{\rm Curr}(F_N)\; \to [0,\infty)}}$ does not extend to a continuous map
${{\overline{{\rm cv}}_N \times \mathcal{S}\; {\rm Curr} (F_N) \to [0,\infty)}}$ .
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