Return words of linear involutions and fundamental groups

Abstract

We investigate the shifts associated with natural codings of linear involutions. We deduce, from the geometric representation of linear involutions as Poincaré maps of measured foliations, a suitable definition of return words which yields that the set of return words to a given word is a symmetric basis of the free group on the underlying alphabet,$A$. The set of return words with respect to a subgroup of finite index$G$of the free group on$A$is also proved to be a symmetric basis of$G$.