Geometry and Transcendence of the Hexponential

Abstract

The modular group $\operatorname{PSL}_2(\mathbb{Z})$ acts on the upper-half plane $\mathbb{HP}$ with quotient the modular orbifold, uniformized by the function $\mathfrak{j} \colon \mathbb{HP}\to \mathbb{C}$. We first show that second derived subgroup $\operatorname{PSL}_2(\mathbb{Z})''$ corresponds to a $\mathbb{Z}^2\rtimes \mathbb{Z}/6$ Galois cover of the modular orbifold by a hexpunctured plane, uniformized by the hexponential map $\operatorname{hexp} \colon \mathbb{HP} \to \mathbb{C} \setminus (\omega_0\mathbb{Z}[j])$, which is a primitive of $C\eta^4$ where $\omega_0\in i\mathbb{R}$ and $C\in \mathbb{R}$ are explicit constants and $\eta$ is Dedekind eta function. We describe the values of the cusp-compactification $\partial \operatorname{hexp}\colon \mathbb{QP}^1\to \omega_0 \mathbb{Z}[j]$. After defining the radial-compactification $\operatorname{Shexp} \colon \mathscr{R} \to \mathbb{R}/(2\pi\mathbb{Z})$, we construct a simple section $\operatorname{InSh} \colon \mathbb{R}/(2\pi\mathbb{Z}) \to \mathscr{S} \bmod{\operatorname{PSL}_2(\mathbb{Z})'}$ where $\mathscr{S} \subset \mathbb{RP}^1$ is a set of numbers whose continued fraction expansions arise from Sturmian sequences, which contains the set $\mathscr{M}$ of Markov quadratic irrationals as those numbers arising from periodic Sturmian sequences. We will show that the values of $\operatorname{InSh}$ are either Markov quadratic irrationals or transcendental. Finally we provide a continued fraction expansion for $\operatorname{hexp}$, and discuss its monodromy.