Automorphic orbits in free groups: words versus subgroups

Pedro Silva (1) , Pascal Weil (2)

1 CMUP - Centro de Matemática, Faculdade de Ciências
2 LaBRI - Laboratoire Bordelais de Recherche en Informatique

We show that the following problems are decidable in a rank 2 free group F_2: does a given finitely generated subgroup H contain primitive elements? and does H meet the orbit of a given word u under the action of G, the group of automorphisms of F_2? Moreover, decidability subsists if we allow H to be a rational subset of F_2, or alternatively if we restrict G to be a rational subset of the set of invertible substitutions (a.k.a. positive automorphisms). In higher rank, we show the decidability of the following weaker problem: given a finitely generated subgroup H, a word u and an integer k, does H contain the image of u by some k-almost bounded automorphism? An automorphism is k-almost bounded if at most one of the letters has an image of length greater than k.

Mots clés: free groups; subgroups; automorphic orbit

Domaines: Théorie des groupes [math.GR]

Pedro Silva, Pascal Weil. Automorphic orbits in free groups: words versus subgroups. International Journal of Algebra and Computation, 2010, 20, pp.561-590. ⟨hal-00324544v3⟩