The geometry of the space of branched Rough Paths

We construct an explicit transitive free action of a Banach space of Holder functions on the space of branched rough paths, which yields in particular a bijection between theses two spaces. This endows the space of branched rough paths the structure of a principal homogeneous space over a Banach space and allows to characterize its automorphisms. The construction is based on the Baker-Campbell-Hausdorff formula and on the Hairer-Kelly map, which allows to describe branched rough paths in terms of anisotropic geometric rough paths.