arXiv 0809.43862008-09math.GR

Automorphic orbits in free groups: words versus subgroups

Pedro Silva, Pascal Weil

备注：: The third version contains additional results, and the material is presented in a different, hopefully clearer fashion
发表信息：: International Journal of Algebra and Computation, 20 (2010) 561-590

英文摘要：

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.

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