Intersection form, laminations and currents on free groups

Ilya Kapovich, Martin Lustig

Research output: Contribution to journalArticlepeer-review


Let F be a free group of rank N ≥ 2, let μ be a geodesic current on F and let T be an ℝ-tree with a very small isometric action of F. We prove that the geometric intersection number <T, μ > is equal to zero if and only if the support of μ is contained in the dual algebraic lamination L2(T) of T. Applying this result, we obtain a generalization of a theorem of Francaviglia regarding length spectrum compactness for currents with full support. We use the main result to obtain "unique ergodicity" type properties for the attracting and repelling fixed points of atoroidal iwip elements of Out(F) when acting both on the compactified outer Space and on the projectivized space of currents. We also show that the sum of the translation length functions of any two "sufficiently transverse" very small F-trees is bilipschitz equivalent to the translation length function of an interior point of the outer space. As another application, we define the notion of a filling element in F and prove that filling elements are "nearly generic" in F. We also apply our results to the notion of bounded translation equivalence in free groups.

Original languageEnglish (US)
Pages (from-to)1426-1467
Number of pages42
JournalGeometric and Functional Analysis
Issue number5
StatePublished - Jan 2010


  • Free groups
  • Geodesic currents
  • Outer space

ASJC Scopus subject areas

  • Analysis
  • Geometry and Topology


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