Parageometric outer automorphisms of free groups

Michael Handel, Lee Mosher

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


We study those fully irreducible outer automorphisms f of a finite rank free group Fr which are parageometric, meaning that the attracting fixed point of φ in the boundary of outer space is a geometric R-tree with respect to the action of Fr, but φ itself is not a geometric outer automorphism in that it is not represented by a homeomorphism of a surface. Our main result shows that the expansion factor of φ is strictly larger than the expansion factor of φ-1. As corollaries (proved independently by Guirardel), the inverse of a parageometric outer automorphism is neither geometric nor parageometric, and a fully irreducible outer automorphism φ is geometric if and only if its attracting and repelling fixed points in the boundary of outer space are geometric R-trees.

Original languageEnglish (US)
Pages (from-to)3153-3183
Number of pages31
JournalTransactions of the American Mathematical Society
Issue number7
StatePublished - Jul 2007

All Science Journal Classification (ASJC) codes

  • Mathematics(all)
  • Applied Mathematics

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