2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/67894Let $F$ be a finitely generated free group. By using Bestvina-Handel theory, as well as some further improvements, the eigengroups of a given automorphism of $F$ (and its fixed subgroup among them) are globally analyzed and described. In particular, an explicit description of all subgroups of $F$ which occur as the fixed subgroup of some automorphism is given.41 pagesGroup TheoryGeometric Topology20E05; 20E36text