An integral weight realization theorem for subset currents on free groups

Ilya Kapovich

Research output: Contribution to journalArticlepeer-review


We prove that if N ≥ 2 and α : FN → φ1(γ) is a marking on FN, then, for any integer r ≥ 2 and any FN-invariant collection of non-negative integral "weights" associated to all sub-trees K of γ of radius ≤ r satisfying some natural "switch" con-ditions, there exists a finite cyclically reduced folded γ-graph δ realizing these weights as numbers of "occurrences" of K in δ. As an application, we give a new, direct, and explicit proof of one of the main results of our paper with Tatiana Nagnibeda (Subset cur-rents on free groups, Geom. Dedicata 166 (2013), 307-348) stating that, for any N ≥ 2, the set SCurrr(FN) of all rational subset cur-rents is dense in the space SCurr(FN) of subset currents on FN. (The proof given in the above-cited paper was indirect and omitted significant details. The proof given here is complete and, we hope, more accessible to the Out(FN) community.) We also answer one of the questions (Problem 10.11) posed in the above-mentioned paper. Thus, we prove that if a nonzero μ € SCurr(FN) has all weights with respect to some marking being integers, then μ is the sum of finitely many "counting" currents corresponding to nontrivial finitely generated subgroups of FN.

Original languageEnglish (US)
Pages (from-to)213-236
Number of pages24
JournalTopology Proceedings
StatePublished - 2017


  • Free groups
  • Geodesic currents
  • Rauzy-de Bruijn graphs.

ASJC Scopus subject areas

  • Geometry and Topology


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