Ordered groups of formal series, and a conjugacy problem

Abstract

Given an ordered field $\mathbb{T}$ of formal series over an ordered field $\mathbf{R}$ equipped with a composition law $\circ \colon \mathbb{T} \times \mathbb{T}^{>\mathbb{R}} \longrightarrow \mathbb{T}$, we give conditions for $(\mathbb{T}^{>\mathbb{R}},\circ)$ to be a group. We show that classical fields of transseries and hyperseries satisfy these conditions. We then give further conditions on $\mathbb{T}$ under which $(\mathbb{T}^{>\mathbb{R}},\circ,<)$ is a linearly ordered group with exactly three conjugacy classes, and solve the open problem of existence of such a group.